News about thallium arsenates(V)

Journal of Alloys and Compounds 820 (2020) 153369

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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

News about thallium arsenates(V) € ger b, Martina Schroffenegger a, Felix Eder a, Matthias Weil a, *, Berthold Sto a c, d Karolina Schwendtner , Uwe Kolitsch a

Institute for Chemical Technologies and Analytics, Division of Structural Chemistry, TU Wien, Getreidemarkt 9/164-SC, A-1060, Vienna, Austria X-ray Centre, TU Wien, Getreidemarkt 9, A-1060, Vienna, Austria c Naturhistorisches Museum Wien, Burgring 7, A-1010, Vienna, Austria d €t Wien, Althanstraße 14, A-1090, Vienna, Austria Institut für Mineralogie und Kristallographie, Universita b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 October 2019 Received in revised form 9 December 2019 Accepted 10 December 2019 Available online 14 December 2019

Systematic phase formation studies in the system Tl/As/O/(H) led to crystal growth of some previously reported and also to numerous novel hydrous and anhydrous phases. Their crystal structures were determined by single crystal X-ray diffraction. This includes the low-temperature forms of TlIH2AsO4 (isotypic with the corresponding phosphate homologues), the hydrogen arsenate TlI2HAsO4 (unique structure), the acidic phases TlIH2AsO4$H3AsO4 and TlI2(H6As4O14)$2H3AsO4 (both unique), the cyclotetrametaarsenate TlI4As4O12 (isotypic with its phosphate homologue), TlI8As8O24 (unique with an arsenate anion of novel constitution), the mixed-valent metaarsenate TlITlIIIAs4O12 (unique) and variscite-type TlIIIAsO4$2H2O. TlIH2AsO4, TlI2HAsO4, TlIH2AsO4$H3AsO4, TlI4As4O12 and TlIIIAsO4$2H2O comprise solely of tetrahedral AsO4 units, TlI2(H6As4O14)$2H3AsO4 and TlI8As8O24 comprise of tetrahedral AsO4 and octahedral AsO6 units, and TlITlIIIAs4O12 contains only AsO6 units. The TlIeO bond lengths span a wide range from ~2.5e3.5 Å associated with coordination numbers between 4 and 12, whereas the TlIII eO bond length are characterized by a narrow range of 2.18e2.32 Å within octahedral coordination spheres. © 2019 Elsevier B.V. All rights reserved.

Keywords: Oxide materials Crystal growth Crystal structure X-ray diffraction Thallium arsenates Mixed-valent phases

1. Introduction Reports on preparations and crystal structure determinations of thallium(I) (thallous) or thallium(III) (thallic) arsenates(V) are scarce. One of the older textbooks on thallium and its compounds [1] compiles only four phases known at that time, viz. TlH2AsO4, Tl2HAsO4 and Tl3AsO4 containing thallium(I) cations as well as TlAsO4$2H2O containing thallium(III) cations. The latter phase was reported to crystallize isotypically with InPO4$2H2O [2], but details of the crystal structure remained unknown so far. Motivated by the occurence of ferroelectricity in solid solutions [(NH4)1-xTlx]H2AsO4 [3], the first systematic phase formation study in the system Tl2O/ As2O5/H2O has been undertaken nearly 60 years ago [4]. Next to TlH2AsO4 and Tl2HAsO4, another phase with composition TlH3As2O7 was reported to exist under these conditions. Some years later, the thermal dehydration behaviour of TlH2AsO4 was investigated, with the acidic thallium(I) diarsenate Tl2H2As2O7 as a

* Corresponding author. E-mail address: [email protected] (M. Weil). https://doi.org/10.1016/j.jallcom.2019.153369 0925-8388/© 2019 Elsevier B.V. All rights reserved.

suggested intermediate phase during thermal treatment, and anhydrous TlI4As4O12 as the final condensation product [5]. However, none of all these phases has ever been structurally characterized in detail, and isotypism with the corresponding phosphates was supposed due to similar X-ray powder diffraction patterns. Although physical properties of TlH2AsO4 were subsequently investigated, in particular dielectric properties and phase transitions ([6], and references therein), the first detailed structure study of any thallium(I) arsenate was reported not until 1987 for the room temperature phase of TlH2AsO4 in its partially deuterated form [7], followed some years later for anhydrous Tl3AsO4 [8]. The current study was devoted to close the gap of knowledge regarding occurence and crystal stuctures of thallium(I) and thallium(III) arsenates(V). Apart from a structural redetermination of the room-temperature polymorph of TlH2AsO4 and its lowtemperature modification, crystal structure determinations of the already known thallium(I) phases Tl2HAsO4 and Tl4As4O12, and of the thallium(III) phase TlAsO4$2H2O are reported here. In addition, crystal structures of the so far unknown thallium(I) phases TlH2AsO4$H3AsO4, Tl2(H6As4O14)$2H3AsO4, Tl8As8O24, and of the mixed-valent phase TlITlIIIAs4O12 are described here for the first

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M. Schroffenegger et al. / Journal of Alloys and Compounds 820 (2020) 153369

time. 2. Materials and methods 2.1. Synthesis and crystal growth Chemicals used (H3AsO4 80%wt; Tl powder, Tl2CO3, HNO3 70%wt) were of p.A. quality and purchased from Merck. 2.1.1. TlIH2AsO4 The dihydrogenarsenate phase was prepared by adding a solution of TlI2CO3 to diluted arsenic acid until the transition point of methyl red was reached. Large lath-like crystals were obtained upon slow evaporation to dryness at room temperature within three days. According to X-ray powder diffraction (XRPD), the material was single-phase. 2.1.2. TlI2HAsO4 Single crystals of the hydrogenarsenate were present as a minority phase in a batch intended for preparation of TlIH2AsO4 containing an excess of about 20%mol of TlI2CO3. TlI2HAsO4 crystals formed as rectangular plates and thus were easily distinguishable from the needle- or lath-like TlIH2AsO4 crystals. Preparation of single-phase microcrystalline TlI2HAsO4 material failed. Evaporation of a stoichiometric 1:1 mixture of TlI2CO3 and H3AsO4 at room temperature led to formation of a mixture of TlIH₂AsO₄ and TlI₃AsO₄ as revealed by XRPD. Changing the evaporation temperatures to 5
C did not change the results whereas small amounts of TlI2HAsO4 could be obtained this way at higher temperatures (75 and 100
C, respectively), however still accompanied by TlI3AsO4 and TlIH2AsO4 as majority phases (together ~ 80%wt). In each of the evaporation studies, the first crystallization product was TlI3AsO4 as revealed by single crystal X-ray diffraction of crystals taken directly from the mother liquor. The highest yield of TlI2HAsO4 was reached by precipitating a TlI2CO3/H3AsO4 mixture (molar ratio Tl:As z 2:1) through dropwise addition of acetone. Again, TlIH₂AsO₄ and Tl₃AsO₄ were the majority phases, and all phases showed poor crystallinity manifested by broad and low-intense reflections. 2.1.3. TlIH2AsO4·H3AsO4 and TlI2(H6As4O14)·2H3AsO4 Thallium powder was treated with an excess of arsenic acid (molar ratio z 10:1) in a teflon-lined steel autoclave at 230
C for one day, resulting in a clear transparent solution. After a few days of standing at room temperature, colourless transparent crystals with different forms could be harvested from the solution. Plate-like crystals of TlIH2AsO4$H3AsO4 had formed mainly on the boundary between the liquid phase and air and were larger than the crystals of TlI2(H6As4O14)$2H3AsO4 that were present as small needles in the liquid. Both types of single crystals were directly immersed in oil for subsequent diffraction experiments. 2.1.4. TlI8As8O24 and TlI4As4O12 Heating of TlIH2AsO4 crystals at 190
C overnight led to the formation of elongated crystals of TlI8As8O24. Repetition of this experiment at 400
C led to crystal growth of TlI4As4O12. Both compounds are hygroscopic, preventing a standard treatment for X-ray powder diffraction. 2.1.5. TlITlIIIAs4O12 This phase was obtained under hydrothermal conditions (7 d, 220
C, autogenous pressure, slow furnace cooling) using Teflonlined stainless steel autoclaves with an approximate filling volume of 1e3 cm3. The autoclaves were filled to about 20e30% of their inner volume with mixtures of H3AsO4,0.5H2O (Alfa, 99.9%), Tl2CO3 and Cr2O3 (Strem Chemicals, 99%) in approximate volume

ratios of H3AsO4,0.5H2O:Tl2CO3:Cr2O3 of 5:1:1 to which two drops of water were added. After the reaction time, hexagonal platelets of TlITlIIIAs4O12 were accompanied by unreacted Cr2O3. 2.1.6. TlIIIAsO4·2H2O Single crystals of TlIIIAsO4$2H2O were prepared according to a literature protocol [2]. 3 g TlIII2O3 were dissolved in boiling concentrated nitric acid (15 ml); the slightly opaque solution was filtered warm through a glass frit and cooled down. Then concentrated arsenic acid was added dropwise (altogether 5 ml) to the light-yellow solution. Water was then added in small portions to the clear solution until the solution began to cloud. The reaction mixture was then heated at ca. 80
C for 2 h. During that time, a light-yellow precipitate formed that contained crystals (typical appearance snubbed bipyramids) of TlIIIAsO4$2H2O with maximum edge lengths of 0.08 mm. An alternative crystal growth method was established by treating the yellow amorphous precipitate, which had formed when the thallium nitrate/arsenate solution was diluted with a higher amount of water, in the mother liquor in a closed vessel at 120
C for one night. The crystals showed the same form and size than those obtained from the other method. 2.2. X-ray Powder Diffraction (XRPD) Samples of the bulk material were ground, fixed with small amounts of petroleum jelly on silicon wafers, and measured with Cu-Ka1,2 radiation in BraggeBrentano geometry on a PANalytical X’PertPro diffractometer (PANAlytical, Almelo, The Netherlands). The system had been recently calibrated with NIST LaB6 standard; X’Celerator multi-channel detector; 2.546
scan length; 25 s exposure time per scan length; 2q range 5e70
; the scans were finally converted into 0.02
step-size bins. 2.3. Single crystal diffraction Diffraction experiments of optically preselected crystals followed standard measurement procedures with corresponding software packages for the diffractometers used [9]. All data sets were corrected for absorption effects, either with numerical methods [10] or with multi-scan approaches [11]. Known or isotypic structures were refined with space group settings and atomic coordinates and labelling taken from previously published data. Novel structures were solved by direct methods [12] or dual space approaches [13], and refined with SHELXL [14]. The crystal structure of TlI4As4O12 was determined from intensity data of a crystal composed of two domains with a random orientation relationship. Both domains were integrated concurrently with overlap information (HKLF5-style file format). Obtaining reliable intensity data for TlIH2AsO4-II and -III was difficult owing to systematic twinning. The monoclinic TlIH2AsO4-II crystals were twinned by the 2[100] operation related to the orthorhombic TlIH2AsO4-I high-temperature phase. On cooling, each twin domain transformed into two triclinic domains, resulting in fourfold TlIH2AsO4-III twins. Integration proved difficult because of a small but distinctly non-zero twin obliquity, which means a high number of reflection overlaps. Ultimately, a non-twinned room temperature crystal was found that transformed into a two-individual twin on cooling and was finally used for structure refinement. The quality of all measured TlI2HAsO4 crystals was poor in terms of intensities and resolution. The Tl2HAsO4 crystal finally used for structure analysis was merohedrally twinned in a 2:3 ratio by 60
rotation with respect to [001]. For the hydrous compounds TlIH2AsO4$H3AsO4, TlI2(H6As4O14)$ 2H3AsO4 and TlIIIAsO4$2H2O, hydrogen atoms could be clearly

M. Schroffenegger et al. / Journal of Alloys and Compounds 820 (2020) 153369

discerned from difference Fourier maps. They were refined with soft OeH distance restraints (0.85e0.90 Å) and individual Uiso(H) parameters. For cases where hydrogen atoms could not be clearly located (TlIH2AsO4-II and -III; TlI2HAsO4), hydrogen atoms were not included in the final model but were considered in the given formulae and other crystallographic parameters. Details on measurement temperatures, diffractometer types, absorption corrections etc. as well as numerical values of the data collections and structure refinements are gathered in Table 1. Further details of the crystal structure investigations may be obtained from The Cambridge Crystallographic Data Centre (CCDC) on quoting the depository numbers listed at the end of Table 1. The data can be obtained free of charge via www.ccdc.cam.ac.uk/ structures. 2.4. Bond valence sum calculations For bond valence sum (BVS) calculations [15], the values provided by Locock and Burns [16] were used for TlIeO bonds, and for TlIIIeO and AsVeO bonds values by Brown and Altermatt [17] were taken. 3. Results and discussion The crystal chemistry of thallium in thallous oxido compounds is dominated by the presence of the 6s2 free electron lone pair E that, in the majority of cases, is stereochemically active [18]. Thus, the resulting coordination polyhedra around TlI are commonly offcentred, and in analogy with compounds comprising neighbouring PbII, the contribution of the ligands can be classified as being holoor hemidirected [19]. As a threshold for the definition of the first coordination sphere of the TlI cations in the crystal structures discussed below, we used a TlIeO bond length of 3.50 Å. This value is based on the sum of the van der Waals radii of Tl and O (1.96 and 1.52 Å [20]) and on BVS calculations [15]. A TlIeO bond of 3.5 Å still accounts for 4.3% of the overall BVS under assumption of an ideal value of þI for monovalent Tl. Given the very broad TlIeO bond lengths distribution between ~2.5 and 3.5 Å in the crystal structures and the concomitant variation in coordination numbers from 4 to 12 (Table 2), it is useful to subdivide TlIeO bonds into “short” bonds less than 3 Å and “long” bonds greater than this boundary up to the maximum value. Judging from the results of BVS calculations, it is obvious that the “long” TlIeO bonds clearly contribute to the overall BVS, leading to more reasonable values for TlI and ligating oxygen atoms. In the following, TlIOx coordination polyhedra are described with the overall coordination number subdivided into “short” (s) and “long” (l) distances to ligating O atoms as [s þ l] (Table 2). In comparison with thallous oxido compounds, the crystal chemistry of thallium in thallic oxido compounds shows a much more uniform distribution of oxido ligands around the TlIII cation, as evidenced by a narrower TlIIIeO bond lengths distribution and a reduction of the coordination number to 6e8, with the octahedral coordination being the most frequently observed. The crystal chemistry of arsenates(V) resembles that of homologous phosphates(V) and thus is dominated by tetrahedral AsO4 units as the principle building block. Derived anions can be fully  2 deprotonated (AsO3 4 ), partly protonated (H2AsO4 ; HAsO4 ), or even fully protonated H3AsO4 groups can be present as arsenic acid adducts in the crystal structures. Corresponding AseO bond lengths follow the general trend that bonds to OH groups are significantly longer than bonds to O atoms only. A similar trend is observed for condensed arsenates(V) where AseO bonds involving bridging O atoms (AseOeAs) are generally longer than those involving terminal O atoms. However, due to the larger radius of arsenic and in strict contrast to phosphates(V), the coordination

3

number of As in arsenates(V) can be expanded from 4 to 6, resulting in the formation of octahedral AsO6 units with much longer AseO bonds (mean: 1.830 Å [21,22] compared to tetrahedral AsO4 (1.687 Å [21]). Such AsO6 groups are rather scarce (less than 5% of all structurally determined arsenates(V) comprise such a unit) and can be present either condensed with AsO4 tetrahedra as part of the anionic framework, or solely in form of condensed anions [22]; isolated AsO6 anions are unknown so far. Graphical representations of the crystal structures [23] in this communication give TlI as blue spheres, without bonds to surrounding O atoms for clarity; TlIII atoms are displayed with lightblue TlIIIO6 coordination octahedra, AsO4 groups as red tetrahedra and AsO6 groups as orange octahedra. H atoms are represented as grey spheres, and O atoms belonging to an OH function (without localisation of the H atom) are yellow. For interpretation of the references to colour in the figure legends, the reader is referred to the Web version of this article. 3.1. TlIH2AsO4 TlIH2AsO4 exhibits three phase transitions, with corresponding phases denoted as I, II and III in order of descending temperature. Phase transition temperatures from high-temperature form I to room-temperature form II and from II to low-temperature form III were determined as 391 and 251 K, respectively. Phase transition I (space group type Pcan) / II (space group type P21/a) is reported to be of second order [6], and is translationengleich with index 2 (t2). The same phase transition sequence I / II is known from the phosphate analogue TlIH2PO4, for which phase transition II (space group type P21/a) / III (space group type C1) was reported as „being slightly of first order“ [24]. Transition II / III can be divided into two separate steps, with P21/n / P1 (t2) and P1 / P1 where the latter is klassengleich with a doubling of the primitive cell (k2); the C-centring in C1 used here is beneficial for direct comparison of the unit cells in I, II and III and causes another doubling of the unit cell of phase III. In the current study, we intended to gain knowledge on structural details of phase I and III. For better comparison, we also reinvestigated the crystal structure of phase II that has been determined previously in its partially deuterated form [7]. The current measurement temperature for phase I (393 K) was close to the reported phase transition temperature (I/II) of 391 K. According to a thermogravimetrical measurement (see supplementary material), polycrystalline TlI2HAsO4 starts to decompose with an on-set of 395 K, i.e. only very slightly above the actual measurement temperature for the diffraction study. Indeed, all crystals under investigation decomposed after a short period of time and thus the obtained data were of very limited value and not sufficient to actually refine the structure. Nevertheless, systematic extinctions indicating space group Pcan, and the obtained lattice parameters (a ¼ 14.69(3), b ¼ 4.672(9), c ¼ 6.67(1) Å) are in agreement with previously reported data [6]. Redetermination of the crystal structure of TlIH2AsO4-II at room temperature confirmed the original structure model [7], however with somewhat higher accuracy and precision. Since a detailed description of the crystal structure is given in the original report and also for the isotypic phosphate analogue [24],1 we discuss here only the most important features. The Tl1 site exhibits a [4 þ 4] coordination polyhedron. Under assumption of similar hydrogen bonding patterns for TlIH2PO4-II (localisation of hydrogen positions

1 In Ref. [7], cell choice 1 (P21/c) was used for space group No. 14 whereas in Ref. [24] cell choice 3 (P21/a) was chosen. In the current refinement of TlIH2AsO4-II we used the same cell choice, labelling and starting coordinates as in Ref. [24].

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Table 1 Details of data collections, structure solutions, and refinements. Formula

TlIH2AsO4-II

Diffractometer Radiation; wavelength/Å Temperature/
C Formula weight Space group (No.)

Bruker APEX-II CCD Bruker APEX-II CCD Mo Ka; 0.71073 Mo Ka; 0.71073

Crystal description Crystal dimensions/ mm3 Formula units Z a/Å b/Å c/Å a/
b/
g/
Volume/Å3 m/mm1 X-ray density/ g$cm3 Rangeqmineqmax/
Range h; k; l Measured reflections Independent reflections Observed reflections [I > 2s(I)] Ri Absorption correction Coef. Of Transmission Tmin; Tmax Structure solution and refinement Number of parameters Extinction coefficient (SHELXTL) Absolute structure parameter R[F2 > 2s(F2)]; wR(F2 all) Goof CSD number

TlIH2AsO4-III

TlI2HAsO4

TlIH2AsO4$H3AsO4

TlI2(H6As4O14)$2H3AsO4

Bruker APEX-II CCD Mo Ka; 0.71073

Bruker APEX-II CCD Mo Ka; 0.71073

Bruker APEX-II CCD Mo Ka; 0.71073

123 345.31

25 548.67 P3 (147) colourless plate 0.22  0.14  0.10

173 1222.36 C2/m (12)

colourless plate 0.22  0.10  0.02

C1 (2) colourless plate 0.22  0.10  0.02

173 487.25 P21/c (14) colourless fragment 0.27  0.26  0.10

colourless plate 0.042  0.03  0.01

4 14.5202(13) 4.6390(9) 6.618(3) 90 92.647(5) 90 445.3(2) 43.522 5.151

16 29.124(7) 9.3141(19) 6.6057(13) 90.06(3) 92.88(3) 90.42(3) 1789.6(7) 43.319 5.127

18 18.712(3) 18.712(3) 7.9699(16) 90 90 120 2416.7(8) 65.954 6.786

4 7.9361(4) 14.2487(7) 7.6166(4) 90 113.104(2) 90 792.20(7) 28.681 4.085

2 19.837(7) 9.223(3) 5.4569(19) 90 97.464(9) 90 989.9(6) 26.315 4.101

2.81 / 29.98 18 / 20; 6 / 6; e8 / 9 4218

1.40 / 32.54 1.26 / 27.88 2.79 / 35.00 2.07 / 30.06 13 / 13; 13 / 14; 9 24 / 24; 22 / 23; 10 / 9 12 / 12; 22 / 22; 12 / 12 27 / 27; 12 / 12; 7 /9 /7 6415 26661 57209 13390

1299

6415

3865

3482

1519

975

5663

1914

3306

1209

0.048 SADABS

e TWINABS

0.149 HABITUS

0.062 SADABS

0.084 SADABS

0.159; 0.495

0.072; 0.269

0.024; 0.178

0.288: 0.752

0.379; 0.746

isomorphous replacement, SHELXL 57

isomorphous replacement, SHELXT, SHELXL SHELXL

SHELXT, SHELXL

SHELXT, SHELXL

219

154

117

65

0.0075(9)

0.00026(5)

e

0.0042(3)

e

e

e

e

e

e

0.0491; 0.1558

0.0652; 0.2000

0.0506; 0.1418

0.0276; 0.0723

0.0299; 0.0445

1.063 1958292

1.049 1958293

0.918 1958288

1.101 1958294

1.024 1958287

25 345.31 P21/a (14)

Formula

TlI8As8O24

TlI4As4O12

TlITlIIIAs4O12

TlIIIAsO4$2H2O

Diffractometer Radiation; wavelength/Å Temperature/K Formula weight Space group (No.)

Bruker APEX-II Mo Ka; 0.71073 173 2618.42 P21/c (14)

Bruker APEX-II Mo Ka; 0.71073 173 1309.16

Nonius Kappa CCD Mo Ka; 0.71073 23 900.42

Crystal description

colourless fragment

P421c (114) colourless plate

Crystal dimensions/mm3 Formula units Z a/Å b/Å c/Å a/
b/
g/
Volume/Å3 m/mm1 X-ray density/g$cm3 Rangeqmineqmax/
Range h; k; l

0.11  0.10  0.05 4 14.2141(6) 7.9569(4) 24.1777(11) 90 104.819(2) 90 2643.5(2) 58.609 6.579 1.48 / 32.88 21 / 21; 12 / 12; e36 / 36

0.05  0.04  0.01 2 7.7555(9) 7.7555(9) 11.3012(14) 90 90 90 679.74(18) 56.983 6.396 3.19 / 30.01 10 / 10; 10 / 10; e15 / 15

P31m (162) colourless pseudohexagonal platelet 0.07  0.01  0.01 1 4.8480(10) 4.8480(10) 11.091(2) 90 90 120 225.75(10) 50.264 6.623 3.67 / 32.50 7 / 7; 5 / 5; 16 / 16

Bruker APEX-II Mo Ka; 0.71073 173 379.32 Pbca (61) light-yellow snubbed bipyramid 0.08  0.08  0.08 8 10.4658(10) 9.1272(9) 10.5106(10) 90 90 90 1004.01(17) 38.659 5.019 3.53 / 37.75 15 / 18; 15 / 15; 14 / 18

M. Schroffenegger et al. / Journal of Alloys and Compounds 820 (2020) 153369

5

Table 1 (continued ) Formula

TlI8As8O24

TlI4As4O12

TlITlIIIAs4O12

TlIIIAsO4$2H2O

Measured reflections Independent reflections Observed reflections [I > 2s(I)] Ri Absorption correction Coef. Of Transmission Tmin; Tmax Structure solution and refinement Number of parameters Extinction coefficient (SHELXTL) Absolute structure parameter R[F2 > 2s(F2)]; wR(F2 all) Goof CSD number

143628 9784 7716 0.117 HABITUS 0.0211; 0.4382 SHELXS, SHELXL

1128 333 318 0.031 HKL Scalepack 0.1268; 0.6334 SHELXS, SHELXL

20783 2746 2207 0.057 SADABS 0.5789; 0.7475 isomorphous replacement, SHELXL

241 e

995 995 978 0.076 TWINABS 0.369; 0.746 isomorphous replacement, SHELXL 42 e

21 0.0084(17)

87 0.00058(4)

e 0.0377; 0.0750 1.098 1958290

0.42(3) 0.0239; 0.0599 1.126 1958289

e 0.0259; 0.0699 1.144 1958286

e 0.0232; 0.0421 1.047 1958291

[24]) and TlIH2AsO4-II (hydrogen positions not localised in the current study), the bond lengths distribution in the AsO4 tetrahedron follows the trend in the corresponding PO4 tetrahedron. For the latter, two hydrogen atoms are disordered around inversion centres and are bonded to O1 and O2, and the third hydrogen atom is ordered and bonded to O3. The corresponding AseOH distances are the longest in the AsO4 tetrahedron, whereas the AseO4 bond is the shortest; here the O4 atom serves as an acceptor for hydrogen bonding but does not carry a hydrogen atom itself. The crystal structure is illustrated in Fig. 1a). The phase transition II / III in TlIH2AsO4 is the same as for the phosphate analogue and thus both phases III are isotypic. The a and b cell parameters of the C-centred setting of phase III are doubled with respect to the primitive setting of phase II. Thus, one out of two translations is lost on cooling, which means that every twin domain can exist in two antiphase domain states, and all atomic sites split into four individual sites in the triclinic polymorph, see Fig. 1b). Accompanied with the lowering of the point group symmetry from 2/m (II) to 1 (III), every twin domain of II may transform into two independent orientation states. In total, crystals of the phase III typically were fourfold twins with twin domains related by the 2[100], 2[010] and 2[001] operations of the mmm point symmetry of the high temperature I phase. This hampered an umambiguous localisation of the hydrogen positions. However, the clear bond lengths distribution separated into two shorter and two longer AseO bonds for each of the four AsO4 tetrahedra (AseO: average 1.67 Å; AseOH: average 1.72 for all tetrahedra) indicates an ordering of the hydrogen positions in analogy with isotypic TlIH2PO4-III. 3.2. TlI2HAsO4 Trigonal TlI2HAsO4 is neither isotypic with its phosphate analogue that adopts a monoclinic structure [25], nor to any other M2HXO4 phase (M ¼ monovalent cation, X ¼ P, As), thus establishing a novel structure type. The complex crystal structure comprises of nine independent Tl, three As and twelve O positions. Two of the Tl sites (Tl8, Tl9) and one O (O10) site are disordered over two sets of sites with a refined occupancy ratio of 0.633(3):0.367(3). Four Tl sites are located on special positions (Tl1 and Tl2 with site symmetry 3; Tl3 and Tl4 with site symmetry 3), all other sites are on general positions. The structral set-up (Fig. 2) can be best described by two types of hexagonal pillars arranged parallel [001]. One pillar comprises I solely of HAs(1)O 4 tetrahedra and is centred by the Tl sites (Tl1, Tl2) situated at the 3 axes on the corners of the unit cell. The other  pillar comprises of HAs(2)O 4 and HAs(3)O4 tetrahedra and is centred by Tl3 situated at the 3 axes. The OH groups in each row

(moved by a 30
rotation relative to [001]) of the pillars point into the same direction. The remaining TlI atoms are either located inbetween the pillars (Tl5, Tl6), or in the space of the pillars (Tl7eTl9). The coordination environments of the nine TlI sites are highly variable in this crystal structure, with coordination numbers ranging from 4 to 12 (Table 2). The most striking feature in the corresponding coordination spheres pertains to the TlIeO bond lengths of the two TlI sites situated on the 3 axes (both with CN ¼ 12). Whereas Tl1 has a set of two pairs of very long TlIeO bonds (average 3.42 Å), Tl2 exhibits two pairs of short and long bonds (average 3.04 Å). In both cases, the BVS values deviate considerably from the expected value of þI (Tl1: 0.61 valence units; Tl2: 1.80 valence units). We therefore checked the nature of these two Tl sites by free refinement of their site occupation factors (s.o.f.) and found full occupancy for theses sites within the standard deviation. Apparently, the empirical bond valence method comes to its limits for these two Tl sites, also taking into account the wellknown inflexible nature of bond-valence parameters in cases of large cations with widely variable cation-oxygen bond lengths. However, all other Tl sites show bond valence sums close to the expected value; refinement of their s.o.f.s also showed full occupancy. The three hydrogenarsenate tetrahedra show the characteristic AseO bond lengths distribution, with the longest bonds to the O atoms that carry the hydrogen atom (O3 for As1; O8 for As2; O11 for As3). This assignment is confirmed by BVS calculations of all oxygen atoms. O3, O8 and O11 have BVS values < 1.25 v. u. while all other O atoms have values between 1.60 and 2.16 valence units. Altough it appears to be likely that those O atoms with a low bond valence sum (O2 1.77 v.u.; O4 1.85 v.u.; O6 1.60 v.u) are involved as acceptor atoms in strong OeH/O hydrogen bonding, the true nature of the hydrogen bonding network remains unknown (poor crystal quality, H atoms not localised, disorder in the structure). 3.3. TlIH2AsO4·H3AsO4 TlIH2AsO4$H3AsO4 (empirical formula TlIH5As2O8) is not isotypic with its phosphate analogue TlIH5P2O8 [26], nor to any other of the known MH5X2O8 structures (M ¼ Rb, X ¼ P [27]; M ¼ Rb, X ¼ As [28]; M ¼ K, X ¼ P [29]; M ¼ NH4, X ¼ P [30]). In the crystal structures of the TleP, RbeP salts and the room-temperature modification of the NH4eP salt, two H2XO4 tetrahedra are linked by a symmetrical hydrogen bond under formation of a centrosymmetric H[H2XO4]2 group with the H atom situated on an inversion centre. On the other hand, in the crystal structures of the KeP, CseP, RbeAs salts and the low-temperature form of the NH4eP salt, the H atom between two H2XO4 tetrahedra is engaged in an asymmetrical hydrogen bond, i.e. it is closer to one of the two

6

M. Schroffenegger et al. / Journal of Alloys and Compounds 820 (2020) 153369

Table 2 Selected bond lengths/Å, averaged (av.) bond length/Å, coordination numbers (CN) and bond valence sums (BVS) for individual atoms as well as details of hydrogen bonding. Phase

av.; CN; BVS

TlIH2AsO4-II Tl1 Tl1 Tl1 Tl1 Tl1 Tl1 Tl1 Tl1 As1 As1 As1 As1

Phase

av.; CN; BVS

TlI2(H6As4O14)·2H3AsO4

O1 O2 O4 O3 O4 O2 O3 O1 O4 O1 O3 O2

2.770 2.905 2.944 2.972 2.981 3.142 3.217 3.494 1.676 1.682 1.689 1.692

(9) (8) (9) (9) (8) (9) (9) (13) (8) (8) (8) (8)

3.05; 8 [5 þ 3]; 0.91

1.685; 4; 5.00

TlIH2AsO4-III

Tl1 Tl1 Tl1 Tl1 As1 As1 As1 As2 As2 As2 As3 As3

O3 O5 O2 O6 O1 O2 O3 O5 O4 O6 O8 O7

2.832 2.868 3.195 3.402 1.631 1.705 1.708 1.661 1.704 1.710 1.764 1.837

As3

O6

(4) (3) (3) (3) (4) (5) (5) (5) (4) (3) (3) (3)

2x 2x 2x 2x

3.07; 8 [4 þ 4]; 0.91

2x

1.688; 4; 5.02

2x 2x 2x

1.696; 4; 4.90

1.883 (3) 2x

1.828; 6; 5.13

Tl1A

O1B

2.791 (11)

D

H

A

DH

H$$$A

D$$$A

DH$$$A

Tl1A

O4C

2.870 (11)

O2

H2

O5

1.69 (7)

2.537 (6)

162 (7)

Tl1A

O4B

2.898 (11)

O3

H3

O8

1.75 (5)

2.583 (5)

167 (6)

Tl1A

O3C

2.936 (12)

O4

H4

O7

1.72 (5)

2.570 (6)

164 (7)

Tl1A

O2A

2.958 (11)

O8

H8

O1

0.88 (6) 0.85 (4) 0.87 (4) 0.87 (5)

1.77 (5)

2.633 (4)

170 (4)

3.196 (14)

TlI8As8O24

Tl1A Tl1A Tl1B Tl1B Tl1B Tl1B Tl1B Tl1B Tl1B Tl1C Tl1C Tl1C Tl1C Tl1C Tl1C Tl1C Tl1C Tl1D Tl1D Tl1D Tl1D Tl1D Tl1D Tl1D Tl1D As1A As1A As1A As1A As1B As1B As1B As1B As1C As1C As1C As1C As1D As1D As1D As1D

O3A O2A O1A O2B O2B O4A O2D O3D O3B O1D O4C O1C O4A O3A O1D O2D O4D O4D O4B O3C O3B O2C O2C O3D O1C O4A O1A O3A O2A O2B O4B O3B O1B O4D O1D O2D O3D O4C O3C O1C O2C

3.222 2.713 2.901 2.957 2.961 2.996 3.086 3.380 2.797 2.837 2.865 2.970 3.056 3.092 3.416 3.417 2.684 2.887 2.910 2.924 3.072 3.189 3.219 3.496 1.664 1.674 1.718 1.719 1.658 1.670 1.718 1.723 1.680 1.682 1.709 1.732 1.669 1.675 1.727 1.732

(13) (11) (11) (13) (12) (12) (11) (14) (11) (10) (13) (12) (13) (12) (13) (14) (12) (11) (13) (13) (12) (11) (11) (16) (11) (11) (13) (11) (10) (11) (12) (12) (12) (11) (11) (10) (10) (11) (13) (10)

2.98; 7 [5 þ 2]; 0.89

3.00; 7 [5 þ 2]; 0.88

3.06; 8 [4 þ 4]; 0.92

3.05; 8 [4 þ 4]; 0.94

1.694; 4; 4.89

1.693; 4; 4.91

1.700; 4; 4.79

1.700; 4; 4.80

TlI2HAsO4 Tl1 Tl1 Tl2 Tl2 Tl3

O4 O2 O2 O3 O12

3.39 3.45 2.62 3.45 3.02

(4) (2) (2) (4) (2)

6x 6x 6x 6x 3x

3.42; 12; 0.61 3.04; 12 [6 þ 6]; 1.80

Tl1 Tl1 Tl1 Tl1 Tl1 Tl1 Tl2 Tl2 Tl2 Tl2 Tl2 Tl2 Tl2 Tl3 Tl3 Tl3 Tl3 Tl3 Tl3 Tl3 Tl4 Tl4 Tl4 Tl4 Tl4 Tl4 Tl4 Tl4 Tl5 Tl5 Tl5 Tl5 Tl5 Tl5 Tl5 Tl6 Tl6 Tl6 Tl6 Tl6

O19 O3 O22 O22 O6 O19 O21 O15 O23 O24 O24 O21 O20 O17 O20 O3 O13 O24 O21 O24 O17 O6 O8 O3 O2 O7 O11 O9 O6 O22 O2 O14 O4 O19 O12 O15 O23 O16 O1 O19

2.565 2.658 2.736 2.935 3.307 3.324 2.673 2.741 2.749 3.130 3.188 3.194 3.233 2.528 2.558 2.749 3.078 3.143 3.190 3.295 2.683 2.796 2.949 2.977 3.085 3.255 3.320 3.340 2.573 2.651 2.915 3.071 3.160 3.171 3.257 2.507 2.576 2.606 2.963 3.435

(7) (7) (7) (7) (8) (7) (7) (7) (7) (7) (7) (7) (8) (7) (7) (7) (7) (7) (7) (7) (7) (7) (6) (7) (7) (7) (7) (7) (7) (7) (7) (7) (7) (8) (7) (7) (7) (7) (7) (7)

Tl7

O24

2.654 (7)

Tl7 Tl7 Tl7 Tl7 Tl7

O7 O21 O9 O20 O11

2.740 2.880 2.931 2.998 3.209

(7) (7) (7) (8) (6)

2.92; 6 [4 þ 2]; 0.97

2.98; 7 [3 þ 4]; 0.94

2.93; 7 [3 þ 4]; 1.10

3.05; 8 [4 þ 4]; 0.94

2.97; 7 [3 þ 4]; 0.99

2.82; 5 [4 þ 1]; 1.02

M. Schroffenegger et al. / Journal of Alloys and Compounds 820 (2020) 153369

7

Table 2 (continued ) Phase

av.; CN; BVS

Tl3 Tl3 Tl4 Tl4 Tl5 Tl5 Tl5 Tl5 Tl6 Tl6 Tl6 Tl6 Tl6 Tl7 Tl7 Tl7 Tl7 Tl7 Tl7 Tl7 Tl8 Tl8 Tl8 Tl8 Tl8 Tl8 Tl9 Tl9 Tl9 Tl9 Tl9 Tl9 As1 As1 As1 As1 As2 As2 As2 As2 As3 As3 As3 As3

O7

3.03 3.45 2.77 3.05 2.48 2.54 2.54 3.15 2.63 2.65 2.68 3.04 3.27 2.52 2.69 2.79 3.23 3.25 3.26 3.43 2.61 2.64 2.75 3.15 3.35 3.38 2.38 2.60 2.64 3.13 3.41 3.43 1.66 1.68 1.68 1.77 1.65 1.65 1.76 1.76 1.59 1.61 1.63 1.78

O10 O10 O6 O1 O9 O5 O3 O5 O9 O1 O8 O11 O4 O2 O4 O9 O3 O11 O3 O7 O10 O12 O5 O8 O5 O6 O12 O7 O1 O9 O11 O4 O2 O1 O3 O5 O7 O8 O6 O12 O10 O9 O11

(2) (4) (3) (3) (3) (3) (4) (4) (3) (3) (3) (2) (2) (4) (2) (4) (4) (4) (2) (4) (2) (3) (2) (4) (2) (3) (3) (2) (2) (3) (3) (2) (4) (2) (3) (4) (4) (2) (2) (3) (2) (3) (4) (2)

3x 3x 3x 3x

Phase

3.17; 9 [3 þ 6]; 0.81 2.91; 6 [3 þ 3]; 0.87

2.68; 4 [3 þ 1]; 1.10

2.85; 5 [3 þ 2]; 0.88

3.02; 7 [3 þ 4]; 0.96

2.98; 6 [3 þ 3]; 0.88

2.93; 6 [3 þ 3]; 1.10

1.70; 4; 4.83

1.71; 4; 4.77

1.65; 4; 5.50

I

Tl H2AsO4·H3AsO4

Tl7 Tl7 Tl8 Tl8 Tl8 Tl8 Tl8 Tl8 Tl8 Tl8 As1 As1 As1 As1 As2 As2 As2 As2 As2 As2 As3 As3 As3 As3 As4 As4 As4 As4 As4 As4 As5 As5 As5 As5 As6 As6 As6 As6 As7 As7 As7 As7 As8 As8

av.; CN; BVS O5 O8 O7 O16 O4 O5 O15 O6 O14 O23 O3 O21 O9 O1 O8 O4 O2 O9 O10 O11 O23 O24 O5 O13 O5 O8 O4 O14 O18 O12 O15 O22 O14 O13 O20 O19 O2 O1 O16 O6 O12 O10 O17 O7

3.352 3.389 2.683 2.735 2.930 2.950 3.227 3.250 3.291 3.401 1.642 1.649 1.736 1.781 1.811 1.818 1.820 1.821 1.836 1.838 1.648 1.650 1.722 1.748 1.812 1.817 1.820 1.828 1.846 1.848 1.648 1.657 1.724 1.769 1.639 1.650 1.734 1.764 1.654 1.668 1.739 1.742 1.653 1.658

(7) (7) (7) (6) (6) (7) (7) (8) (7) (8) (7) (7) (7) (7) (6) (6) (7) (6) (7) (7) (7) (7) (7) (6) (6) (6) (6) (6) (7) (7) (7) (7) (7) (7) (7) (7) (7) (7) (7) (7) (7) (6) (7) (7)

3.02; 8 [5 þ 3]; 1.02

3.06; 8 [4 þ 4]; 0.95

1.702; 4; 4.83

1.824; 6; 5.14

1.692; 4; 4.93

1.829; 6; 5.09

1.700; 4; 4.84

1.698; 4; 4.89

1.701; 4; 4.82

As8

O11

1.734 (7)

Tl1

O5

2.702 (2)

As8

O18

1.746 (6)

Tl1

O2

2.965 (3)

TlI4As4O12

Tl1 Tl1 Tl1 Tl1 Tl1 Tl1 As1 As1 As1 As1 As2 As2 As2

O7 O8 O3 O4 O7 O4 O1 O2 O3 O4 O5 O6 O8

2.998 3.004 3.016 3.060 3.248 3.468 1.649 1.690 1.699 1.700 1.654 1.664 1.720

Tl1 Tl1 Tl1 Tl1 Tl2 Tl2 Tl2 Tl2 Tl2 As1 As1 As1 As1

As2

O7

1.720 (3)

D

H

A

DH

H$$$A

D$$$A

DH$$$A

Tl1

O2

2.247 (4) 6x

2.25; 6; 3.13

O2

H2

O6

0.87 (9)

2.470 (4)

164 (10)

Tl2

O1

2.840 (4) 6x

2.84; 6; 0.97

O3

H3

O5

0.87 (3)

2.576 (4)

175 (9)

As1

O1

1.794 (2) 3x

O4

H4

O1

0.86 (3)

1.62 (3) 1.71 (2) 1.62 (3) 1.71 (3)

2.477 (4)

169 (10)

As1

O2

1.877 (3) 3x

165 (10)

III

O7

H7

O1

(3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (2) (2) (3)

3.06; 8 [2 þ 6]; 0.90

1.685; 4; 5.05

1.690; 4; 4.96

0.88 (5)

2.568 (3)

O2 O3 O2 O1 O3 O2 O2 O1 O3 O2 O3 O1 O1

2.639 2.702 3.199 3.370 2.892 3.027 3.061 3.408 3.416 1.638 1.644 1.742 1.769

(8) (8) (8) (8) (8) (8) (8) (8) (8) (8) (8) (8) (8)

2x 2x 2x 2x 2x 2x 2x 2x 2x

1.698; 4; 4.86

2.98; 8 [4 þ 4]; 1.18

3.16; 10 [2 þ 8]; 0.92

1.698; 4; 4.87

TlITlIIIAs4O12

1.836; 6; 5.02

Tl AsO4·2H2O (continued on next page)

8

M. Schroffenegger et al. / Journal of Alloys and Compounds 820 (2020) 153369

Table 2 (continued ) Phase O8

av.; CN; BVS H8

O6

0.87 (3)

1.75 (4)

2.602 (4)

Phase 164 (9)

av.; CN; BVS

Tl

O2

2.183 (2)

Tl Tl Tl Tl Tl As As As As

O3 O4 O1 O2W O1W O3 O1 O4 O2

2.202 2.206 2.231 2.232 2.320 1.686 1.687 1.690 1.691

D

H

A

D-H

H…A

D…A

D-H … A

O1W

H1

O1

2.10 (2)

2.908 (3)

162 (5)

O1W

H2

O3

1.93 (2)

2.737 (4)

159 (5)

O2W

H3

O4

1.76 (2)

2.601 (4)

172 (6)

O2W

H4

O2

0.85 (1) 0.85 (1) 0.85 (1) 0.85 (1)

1.84 (3)

2.624 (4)

154 (5)

(3) (2) (2) (2) (3) (2) (2) (2) (2)

2.23; 6; 3.32

1.689; 4; 4.95

Fig. 2. The crystal structure of Tl2HAsO4 in a projection along [001]. Only the major parts of disordered atoms are shown for clarity.

3.4. TlI2(H6As4O14)·2H3AsO4 Fig. 1. The crystal structures of TlIH2AsO4-II (a)) and TlIH2AsO4-III (b)) in a projection along [010].

tetrahedra and thus defines distinct H3XO4 and H2XO4 groups. Hence the latter compounds can be considered as phosphoric or arsenic acid adducts, respectively. The same is valid for TlIH2AsO4$H3AsO4 that comprises of H2AsO 4 (As2) and fully protonated H3AsO4 (As1) tetrahedra. The two types of tetrahedra are arranged in double layers parallel (100) (Fig. 3). Very strong to strong hydrogen bonds (O/O ¼ 2.47e2.60 Å; Table 2) between the dihydrogenarsenate units and arsenic acid units are present in the crystal structure. A similar layered arrangement is realized in the crystal structures of compounds with compositions M(H2AsO4)2(H3AsO4)2 (M ¼ Mg, Mn, Co, Ni), M(HAsO4)(H3AsO4)(H2O)0.5 (M ¼ Mn, Cd) and Zn(HAsO4)(H3AsO4) that comprise H3AsO4 tetrahedra in their structures [31]. The TlI cations (Tl1) are located between the double layers and are surrounded by eight O atoms in a [2 þ 6] coordinatiom from adjacent layers in form of an irregular polyhedron.

TlI2(H6As4O14)$2H3AsO4 (empirical formula TlH6As3O11) comprises As[4] and As[6] in form of a finite centrosymmetric (H6As4O14)2- anion (point group symmetry 2/m). This anion is composed of two corner-sharing AsO6 octahedra (As3) that are bridged by two AsO4 tetrahedra (As2) via corners. The four terminal O atoms (O8 and its symmetry-related counterparts) belonging to the As2O10 dimer are protonated, as well as two of the four terminal O atoms (O4 and its symmetry-related counterpart) of the AsO4 tetrahedra (Fig. 4). The set-up of the (H6As4O14)2- anion in the thallous compound is very similar to that in BaH6As4O14 [32], with the exception of the OH group at the tetrahedron being in a different position. The As4O14 core present in both types of (H6As4O14)2- anions is the predominant structure unit in arsenates with monovalent cations containing AsO4- and AsO6- subunits [22]. In the crystal structure of TlI2(H6As4O14)$2H3AsO4, the (H6As4O14)2anions are stacked into rows parallel [001]. Fully protonated tetrahedral H3AsO4 groups (As1; point group symmetry 2) are situated in-between the rows and are connected to the (H6As4O14)2anions by an intricate network of strong to medium-strong

M. Schroffenegger et al. / Journal of Alloys and Compounds 820 (2020) 153369

Fig. 3. The crystal structure of TlIH2AsO4$H3AsO4 in a projection along [001].

9

Fig. 5. The crystal structure of TlI8As8O24 in a projection along [010].

l and TlIH2AsO4 at 190
C, in contrast to the investigations of Dosta Kocman who reported TlI2H2AsO7 as the dehydration product of TlIH2AsO4 at 200
C, as revealed by a thermogravimetric measurement [5]. Tl8As8O24 has an empirical formula of TlAsO3 and hence formally represents a metaarsenate(V) with a Tl:As ratio of 1:1. As distinguished from alkali metaarsenates(V) that crystallize either with ring (cylo-metaarsenates) or chain anions (catenametaarsenates) comprising of tetrahedral AsO4 units [33], the finite oxoarsenate(V) anion in the crystal structure of TlI8As8O24 has a novel constitution and is made up from AsO4 and AsO6 groups. The 8 As8O24 anion exhibits point group symmetry 1. It is built from the same type of As4O14 core that is present in the crystal structrure of TlI2(H6As4O14)$2H3AsO4. Two additional pairs of diarsenate groups As2O7 complete the anion in the structure of TlI8As8O24 by sharing corners with the four terminal O atoms of the As4O14 core. The only other metaarsenate anion of a monovalent cation comprising of AsO4 and AsO6 groups is found in AgAsO3 [34]. However, the nature of this anion is polymeric, comprising of chains of corner-sharing AsO6 octahedra flanked by AsO4 tetrahedra. In the crystal structure of TlI8As8O24, the isolated anions are organised in rows extending parallel [010]. The eight unique TlI atoms are located inbetween neighbouring anionic groups (Fig. 5). They show a very variable coordination to the O atoms of the anions, with coordination numbers ranging from 5 to 8. 3.6. TlI4As4O12 Fig. 4. The crystal structure of TlI2(H6As4O14)$2H3AsO4 in a projection along [001].

hydrogen bonds (O/O ¼ 2.54e2.64 Å; Table 2). Tl1 is located at an inversion centre and is connected to eight O atoms in a [4 þ 4] coordination by O atoms. The TlIO8 polyhedra share common edges and are concatenated into rows extending parallel [010]. 3.5. TlI8As8O24 Tl8As8O24 has formed under mild thermal treatment of

Tl4As4O12 has the same empirical formula as TlI8As8O24 („TlIAsO3“) and represents the high-temperature polymorph obtained at higher reaction temperatures of 400
C. In agreement l and Kocman [5], the final condensation with the findings of Dosta product of the thermal dehydration is TlI4As4O12, comprising of finite As4O4 12 ring anions. Hence the structural phase transition from TlI8As8O24 to TlI4As4O12 is of the reconstructive type. TlI4As4O12 represents a cyclo-tetrametaarsenate made up of cornersharing AsO4 units and is isotypic with TlI4P4O12 [35]. Other representatives for this tetragonal structure type are unknown so far. The two unique TlI sites are both located on twofold rotation axes

10

M. Schroffenegger et al. / Journal of Alloys and Compounds 820 (2020) 153369

Fig. 6. The crystal structure of TlI4As4O12 in a projection along [100]. Colour code as in Fig. 3. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

and exhibit coordination numbers of eight for Tl1 [4 þ 4] and ten [4 þ 6] for Tl2. The cyclo-tetrametaarsenate anion As4O4 12 has point group symmetry 4 and is built by one crystallographically independent AsO3 group, with the usual AseO bond lengths distribution between the two pairs of terminal and bridging O atoms (average 1.64 and 1.76 Å, respectively). The isolated ring anions are organised in pseudo-hexagonally arranged rows extending parallel to [100] with the two types of TlI atoms situated in-between. The crystal structure of TlI4As4O12 is depicted in Fig. 6. 3.7. TlITlIIIAs4O12 TlITlIIIAs4O12 was obtained serendipitously during phase formation studies of a prospective phase TlCrIIIAs2O7 adopting the triclinic TlIInAs2O7 structure type [36]. So far, TlITlIIIAs4O12 is the only arsenate(V) containing Tl in two different valence states. According to the Robin-Day classification [37], TlITlIIIAs4O12 belongs to class I with valences clearly located on a single site. To the best of our knowledge, next to the oxide Tl4O3 [38] only four other oxido Table 3 Comparison of unit cell parameters and symmetry of TlITlIIIAs4O12 with related oxoarsenates(V). Compound I

III

SG

a/Å

c/Å

V/Å3

Z

Reference this work

Tl Tl As4O12

P31m

4.848(1)

11.091(2)

225.75(8)

1

CaAs2O6

P31m

4.8258(12)

5.0824(7)

102.50

1

[43]

MnAs2O6

P31m

4.7956(9)

4.6923(6)

93.46

1

[44]

CoAs2O6

P31m

4.7768(12)

4.4968(8)

88.86

1

[44]

NiAs2O6

P31m

4.7585(7)

4.4349(4)

86.97

1

[44]

PdAs2O6

P31m

4.8196(1)

4.6646(1)

93.84

1

[45]

CdAs2O6

P31m

4.8269(10)

4.866(1)

98.18

1

[46]

HgAs2O6

P31m

4.8482(2)

4.9798(10)

101.37

1

[47]

Hg2As2O6

P31m

4.8411(2)

7.5961(9)

154.17

1

[47]

PbAs2O6

P31m

4.8698(12)

5.4837(2)

112.62

1

[43]

a-SrAs2O6 b-SrAs2O6

P31m P63/mcm

4.8580(10)

5.4260(10)

110.90

1

[48]

LiAsO3

R3

4.8530(10) 4.808(3)

10.867(2) 14.21(2)

221.65 284.48

2 6

[48] [49]

compounds with mixed-valent TlI/III have been structurally characterized, viz. TlPd3O4 [39], Tl4(IO3)6 [40], Tl2(CrO4) [41] and Tl3(OH)(SO4)2 [42]. In the crystal structure of TlITlIIIAs4O12, Tl1 represents the TlIII atom and Tl2 the TlI atom. Bothare situated on sites with symmetry 3.m. Tl1 has a nearly undistorted octahedral oxygen environment with a bond length of 2.247 Å, in very good agreement with the mean TlIII[6]eO bond length of 2.228 Å [21]. Tl2 exhibits a coordination by six O atoms at an equal distance of 2.84 Å, likewise in an octahedral coordination sphere but with much greater angular distortions (OeTleO angles vary between 69 and 112
). Hence Tl2 represents a rare case where the stereochemical activity of the lone pair E does not have a predominant role in the stereochemistry of TlI. TlITlIIIAs4O12 crystallizes in a unique structure type that is closely related to common metaarsenates of the general formula MIIAs2O6 (MII ¼ Ca, Mn, Co, Ni, Cd, Hg, Pb, Pd, Sr) crystallizing in the trigonal PbSb2O6 structure type, except for the b-polymorph of SrAs2O6 that adopts hexagonal symmetry (Table 3). The basic building arrangement consists of layers of edge-sharing AsO6 octahedra that are stacked along [001]. The differences arise solely by the atoms that sit in-between these layers and interconnect them. These are either MI and MIII cations in the case of TlITlIIIAs4O12, MII ions in the case of MIIAs2O6, or pairs of covalently bonded HgI atoms in Hg2As2O6 (all with Z ¼ 1). The structural difference between TlITlIIIAs4O12 and MIIAs2O6 is due to the presence of TlIII and TlI. The two positions are therefore not identical anymore (excluding a formulation as “TlAs2O6”), and the unit cell is doubled along c. By comparison, the b-polymorph of SrAs2O6 exhibits Z ¼ 2 and is likewise characterized by a doubling of the c axis. In the b-SrAs2O6 structure, the SrII atom has a trigonal-prismatic environment instead of an octahedral one in the PbSb2O6-type aform. In the structure of Hg2As2O6 (Z ¼ 1) the metal position M in MIIAs2O6 (Z ¼ 1) is replaced by a pair of covalently bonded HgI atoms (Hg2þ 2 dumbbell) - this also leads to an increase of the unit cell with a considerably longer unit cell parameter c. One of the LiAsO3 polymorphs [49] is again closely related to the MIIAs2O6 structures, but due to the replacement of large MII ions by the much smaller LiI ion, a different structural set-up is realized, adopting the ilmenite structure type. Here, LiI atoms are displaced alternatingly in þz and ez direction, and therefore the m<120> operations of the Li layers are lost. Moreover, the subsequent Li layers are displaced in the (001) plane leading to an R-centred structure. Nevertheless, the basic building unit - layers of edge-sharing AsO6 octahedra - remains unchanged in LiAsO3. In all other structure types (Table 3), the MI, MII or MIII ions sit above the position of the voids in the arsenate(V) layers. The small LiI ions basically form their own layers of edge-sharing LiO6 octahedra whereby AsO6 and LiO6 octahedra share faces. This unfavoured situation is compensated by a relatively strong distortion of the LiO6 octahedra; the LiI atom moves as far as possible away from this face. Another difference to MIIAs2O6 structures pertains to the [001] stacking of AsO6 layers in LiAsO3. The arsenate(V) layers are not stacked exactly on top of each other anymore, and every second layer is shifted in such a way that an AsO6 octahedron is positioned on an empty position (the centre of each six-ring) in layer one. The crystal structures of TlITlIIIAs4O12, PbSb2O6-type MIIAs2O6 and LiAsO3 are compared in Fig. 7. 3.8. TlIIIAsO4·2H2O TlIIIAsO4$2H2O crystallizes in the orthorhombic variscite structure type for which numerous representatives are known. All atoms in the TlIIIAsO4$2H2O structure are located in general positions, leading to distorted octahedral TlIIIO6 units but rather regular tetrahedral AsO4 units. Consequently, the TlIIIeO bond lengths

M. Schroffenegger et al. / Journal of Alloys and Compounds 820 (2020) 153369

11

structure with channels running parallel [010] (Fig. 8). The hydrogen atoms point into these channels and are bonded with three hydrogen bonds of medium (O/O ¼ 2.60e2.74 Å) and one of weak strength (2.91 Å) to adjacent framework O atoms. 3.9. Summary of AseO and TleO bond length

Fig. 7. The crystal structures of TlITlIIIAs4O12 (a)); MIIAs2O6 (b)), with data from the a-Sr member [48] and with Sr sites in blue); LiAsO3 (c)) [49], with Li sites in blue). The left part shows the stacking of arsenate layers along [001] and the right part a view onto the layers. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

The averaged AseO bond lengths for each AsO4 tetrahedron and AsO6 octahedron, respectively, in the refined nine structures are given in Table 2. The individual averaged values scatter slightly (AsO4: range 1.65e1.72 Å; AsO6: 1.824e1.836 Å). The overall mean of the 20 independent tetrahedral AsO4 groups in the nine structures is 1.689 Å, a value in very good accordance with that of 1.687 Å reported in literature [21]. A very recent statistical evaluation of 2  AseO bond lengths subdivided into AsO3 4 , HAsO4 , H2AsO4 and H3AsO4 groups revealed the following average values [50]: 1.667(18) Å for AseO bonds to nonprotonated O atoms, 1.728(19) Å for AseOH bonds in HAsO2 4 groups, 1.714 (12) Å for AseOH bonds in H2AsO 4 groups and 1.694(16) Å for AseOH bonds in H3AsO4 groups, in good agreement with individual values collated in Table 2. The same is valid for the four independent octahedral AsO6 groups present in three of the structures. The averaged AseO bond length of 1.829 Å matches the reported mean value of 1.830 Å in structures of several inorganic oxoarsenate compounds containing AsO6 units [22]. The coordination numbers of the 25 independent TlI cations in the nine structures range from 4 to 12, spanning a wide range of TlIeO bond lengths from ~2.5e3.5 Å. The mean TlIeO distances in the TlIOx polyhedra (Table 2) are in reasonable agreement with literature values for the different coordination numbers: CN ¼ 3, 2.52(5) Å; 4, 2.73(23) Å; 5, 2.84(30) Å; 6, 2.89(19) Å; 7, 2.98(23) Å; 8, 2.99(19) Å; 9, 2.99(19) Å; 10, 3.10(22) Å; 11, 3.13(26) Å; 12, 3.20(29) [21]. 4. Conclusions The results of phase formation studies in the system Tl/As/O/(H) supplement the knowledge on thallium oxoarsenates(V) and their crystal structures. Based on previous investigations, the supposed isotypism to analogous phosphate(V) phases was confirmed for the low-temperature form of TlIH2AsO4, the cyclo-tetrametaphosphate TlI4P2O12, and variscite-type TlIIIAsO4$2H2O, whereas TlI2HAsO4 is not isotypic with TlI2HPO4 but crystallizes in a unique structure type. The previously reported acidic diarsenate phases Tl2H2As2O7 and TlH3As2O7 could not be isolated during the current study. The acidic phases TlIH2AsO4$H3AsO4 and TlI2(H6As4O14)$2H3AsO4 have never been reported before, just like TlI8As8O24 that represents an intermediate product of the thermal treatment of TlIH2AsO4, and the mixed-valent polyarsenate(V) TlITlIIIAs4O12. Each of the latter four phases crystallizes in a novel structure type and thus add details to the crystal chemistry of thallium and oxoarsenate(V) compounds.

Fig. 8. The crystal structure of TlAsO4$2H2O in a projection along [010].

scatter more widely (2.18e2.32 Å, with the two TleOwater bonds being the longest) in comparison to those of mixed-valent TlITlIIIAs4O12 with its highly symmetric TlIIIO6 polyhedron. Nevertheless, the average TlIIIeO bond length of 2.29 is in excellent agreement with the reported mean value of 2.228 Å [21]. In the crystal structure of TlAsO4$2H2O, isolated octahedral and tetrahedral building blocks share corners to build up a framework

Author contributions M.W., B.S., K.S. and U.K. conceived and designed this study and also performed structure analyses based on X-ray diffraction methods. M.S., F.E., K.S. and M.W. performed crystal growth experiments; K.S., U.K., B.S. and M.W. prepared the draft manuscript, and M.W. edited and revised the final version. Funding This research received no external funding.

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M. Schroffenegger et al. / Journal of Alloys and Compounds 820 (2020) 153369

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