High-temperature in situ neutron powder diffraction study of monoclinic Tl2Ba2CuO6+δ

International Journal of Inorganic Materials 2 (2000) 533–541

High-temperature in situ neutron powder diffraction study of monoclinic Tl 2 Ba 2 CuO 61d D.R. Pederzolli a

a,b ,1

, J.P. Attfield

a,b ,

*

Interdisciplinary Research Centre in Superconductivity, University of Cambridge, Madingley Road, Cambridge CB3 0 HE, UK b Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1 EW, UK Dedicated to Professor Bernard Raveau on the occasion of his 60th Birthday

Abstract The monoclinic form of the Tl 2 Ba 2 CuO 61d superconductor (F112 /m symmetry) has been studied using high-resolution hightemperature in situ time-of-flight neutron powder diffraction. The amount of excess oxygen controls both the crystal structure and ˚ b55.5026 A, ˚ c523.1523 A, ˚ g 590.127 8, d 50.15) is non-superconducting, electronic properties: the as-prepared material (a55.4412 A, ˚ b55.5288 A, ˚ c523.3076 A, ˚ g 590.034 8, d 50.11) is superconducting below whereas the 700 K vacuum-annealed sample (a55.4874 A, T c 573 K. Thermal variations of atomic c-axial displacements and bond valence sums demonstrate an electronic charge transfer from the TlO layers to the CuO 2 planes. The peak widths of the Bragg reflections indicate microscopic strains in the c-axial direction, due to disordering of interstitial oxygen in the Tl 2 O 21d bilayer, and in the ab-plane, due to the monoclinic and orthorhombic distortions, or static displacements of the Tl and O atoms in the TlO layers. Superstructural diffraction peaks due to an incommensurate structural modulation decrease in intensity with oxygen loss over the monoclinic-to-orthorhombic progression.  2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Oxides; C. Neutron scattering; D. Crystal structure; Superconductivity

1. Introduction Tl 2 Ba 2 CuO 61d (Tl-2201) is a useful material for studying the intrinsic transport properties of high critical temperature (T c ) superconductors [1,2], since it can be prepared as high-quality single crystals with low residual resistivities. The overdoped regime of the superconducting phase diagram has been studied by lowering the T c from 90 K to below 4 K through the addition of oxygen [3]: the critical magnetic field scale is rapidly decreased and the normal metallic state can be studied to mK temperatures [2]. Pure anisotropic d x 2 – y 2 pairing symmetry in tetragonal Tl-2201 films has recently been evidenced using scanning SQUID microscopy [4,5], emphasising the important role of Tl-2201 in the development of a theoretical model for high-temperature superconductivity. *Corresponding author. Tel.: 144-1223-336-332; fax: 144-1223-336362. E-mail address: [email protected] (J.P. Attfield). 1 Present address: Johnson Matthey plc., Orchard Road, Royston, Herts SG8 5HE, UK.

Tl 2 Ba 2 CuO 61d is the first member of the Tl 2 Ba 2 Ca n21 Cu n O 412n series and has one of the highest reported T c values of any single copper oxide layered compound, with T c (max)593 K [6,7]. Structurally, it is perhaps the most interesting of the thallium superconductors as it exhibits two distinct macroscopic structural symmetries, tetragonal and orthorhombic, unlike other homologues which invariably display tetragonal symmetry. Flux-grown single-crystals are always tetragonal and as a consequence the orthorhombic variant has been rather less studied. Physical measurements [8] and a resonant X-ray diffraction study [9,10] have shown that the two polymorphs possess slightly different compositions: the orthorhombic form is essentially thallium-stoichiometric Tl 2 Ba 2 CuO 61d whereas the tetragonal form contains a Cu / Tl substitutional defect, giving the composition (Tl 1.9 Cu 0.1 )Ba 2 CuO 61d . We have recently reported a new monoclinic form of superconducting Tl 2 Ba 2 CuO 61d , evidenced by high-resolution time-of-flight neutron and synchrotron X-ray powder diffraction [11], and a hightemperature in situ X-ray powder diffraction study of the changes in microscopic and macroscopic strains at the

1466-6049 / 00 / $ – see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S1466-6049( 00 )00073-8

534

D.R. Pederzolli, J.P. Attfield / International Journal of Inorganic Materials 2 (2000) 533 – 541

monoclinic to orthorhombic and orthorhombic to tetragonal phase transitions [12]. These have established Tl-2201 as a unique high-T c material displaying three distinct macroscopic structural symmetries. Here we report a highresolution high-temperature in situ time-of-flight neutron powder diffraction study of monoclinic Tl-2201, the first of its kind for thallium or mercury cuprate superconductors due to the difficulties caused by the volatility and toxicity of the heavy metals in these materials. The study has allowed a complete determination of the structural and compositional changes accompanying the monoclinic-toorthorhombic progression.

2. Experimental A 9-g polycrystalline sample of monoclinic Tl 2 Ba 2 CuO 61d was synthesised from mixtures of highpurity Tl 2 O 3 and CuO (.99.9%) and BaO 2 (.98%) powders, using the starting composition Tl:Ba:Cu5 2.10:2.00:1.06. The powders were ground into a paste under acetone using an agate pestle and mortar and pressed into three pellets, each 13 mm in diameter, under a pressure of 5.0 tonne cm 22 . The pellets were placed in a gold foil tube, crimped at the edges to minimise volatilisation of thallium oxides at elevated temperatures. The reaction profile was 8108C (20 h), 8208C (12 h), 8208C (12 h) under flowing oxygen gas throughout. The grinding, pelletting and sealing procedure was repeated after each heating cycle. The sample was quenched to room temperature after the first two heat treatments but slow-cooled (58C min 21 ) after the final treatment. High-resolution time-of-flight neutron powder diffraction studies were carried out on the HRPD diffractometer at ISIS, Rutherford Appleton Laboratory, UK. Eight grams of the as-prepared material were reground and placed in a cylindrical thin-wall vanadium can (11 mm diameter, 25 mm sample height) inside a resistively heated furnace. The sample environment was evacuated and diffraction patterns were recorded at seven temperatures: 296 K (room temperature), 400 K, 500 K, 550 K, 600 K, 650 K and 700 K. The sample temperature was monitored via a digital thermocouple thermometer and was maintained to 62.0 K of the desired temperature for T #600 K and to 61.0 K for T .600 K during data collection. The total neutron flux incident on the sample at each temperature was the same (16661 mA over 5–7 h). Data from the backscattering (1608#2u #1768) and 908 (878#2u #938) detector banks, 24 23 for which the Dd /d resolutions are 4310 and 2310 , respectively, were corrected for instrumental background contributions and normalised to the incident neutron flux distribution using a vanadium spectrum. The profiles from both data banks were fitted simultaneously by the Rietveld method [13] using the GSAS package [14]. The magnetic susceptibilities of the as-prepared material

and the residue from the high-temperature diffraction experiment were measured using a Lakeshore 7000 AC susceptometer. The data were corrected for background and demagnetisation. The as-prepared material was nonsuperconducting above 4 K but the residue from the high-temperature diffraction experiment was superconducting below T c (onset)573 K.

3. Results

3.1. Structural refinement The principal diffraction peaks in the room-temperature pattern could be indexed to monoclinic Tl 2 Ba 2 CuO 61d (space group F112 /m) [11], but several additional peaks, the most intense occurring at d-spacings of 1.786, 2.464 ˚ were observed in both profiles. Some of the and 3.064 A, less intense peaks were attributed to traces of the impurity CuO (space group C12 /c1 [15]) and fitted in the refinement. Of the remaining additional peaks, the intensities were ,4% relative to the most intense Tl 2 Ba 2 CuO 61d ¯ reflection (220 / 220 doublet), but the relative intensity of ˚ was ca. 20%. The position of the the peak at d52.466 A ˚ is similar, but its Tl 2 Ba 2 CuO 61d (204 ) peak (d52.462 A) calculated intensity is more than a factor of two smaller. ˚ have been The three peaks at d51.786, 2.464 and 3.064 A observed previously in an oxygen-rich sample of ‘orthorhombic’ Tl-2201 by synchrotron X-ray diffraction [10], and are due to an incommensurate structural modulation. Although the intensities of these peaks decreased with increasing temperature, the two most intense of these were removed from all refinements via excluded regions from ˚ and 3.02–3.10 A. ˚ 2.41–2.58 A The first refinement (T5296 K) was carried out using the monoclinic F112 /m structure reported previously as a starting model [11]. The nuclear scattering lengths used 212 were 0.8790, 0.5250, 0.7718 and 0.5805310 cm for Tl, ˚ Ba, Cu and O. Data ranges of 38–116 ms (0.78–2.41 A) ˚ and 40–122 ms (1.14–3.51 A) from the backscattering and 908 detector banks were used in the refinements 2 . Diffraction peaks in both profiles were fitted using the exponential pseudo-Voigt function with full widths at half maximum for Gaussian and Lorentzian broadening modeled, respectively, by:

GG 5 d(s 12 1 s 22 d 2 )1 / 2

(1)

GL 5 g d 1 g d cos f

(2)

for refineable coefficients s1 , s2 , g and g 9, and an anisotropic broadening axis of [001 ]. Microscopic strains

2

The data ranges were adjusted so that an equal number of Bragg reflections were used for each refinement.

D.R. Pederzolli, J.P. Attfield / International Journal of Inorganic Materials 2 (2000) 533 – 541

parallel (S/ / ) and perpendicular (S' ) to the broadening axis were calculated from the refined Lorentzian parameters for the profile for the high-resolution bank (2u 51708, C5 48229.96) using the expressions: 1 S/ / 5 ] (g 1 g 9) C

(3)

g S' 5 ] C

(4)

The thermal factor of the interstitial O(4) atom was unstable due to correlation with its low site occupancy. Consequently, the temperature factors of O(4) and the other disordered oxygen site, O(3), were constrained to be equal, allowing the occupancy of the O(4) site to be refined freely. The degree of substitution of Cu at the Tl site was not found to be significant; refined site occupancies of Tl and Cu(Tl) were within 1 E.S.D. of 1.00 and 0.00, respectively, so this defect was not modeled in the final refinement. For refinements at the higher temperatures, the final model of the refinement at the previous temperature was used as the starting model. Excellent fits to the data were obtained from both backscattering and 908 banks, with overall R wp values ranging from 5.0 to 7.1%. The principal refinement results are given in Table 1, and

535

observed, calculated and difference profiles for data collected at 700 K are shown in Fig. 1.

3.2. Incommensurate structural modulation in monoclinic Tl2 Ba2 CuO61d The additional peaks in the neutron diffraction patterns, which could not be indexed to Tl 2 Ba 2 CuO 61d , CuO or other impurities, are due to the incommensurate structural modulation with the wavevector: q 5 qx a* 1 qy b* 1 qz c*

(5)

and have reciprocal lattice vectors given by: d* 5 (h6mqx ) a* 1 (k 1 mqy ) b* 1 (l 1 mqz ) c*

(6)

Using this expression and the propagation vector k60.07, 0.22, 1l [10], four well-resolved superstructural peaks in the room-temperature diffraction pattern at d5 ˚ could be indexed as 1.786, 1.804, 2.464 and 3.064 A hklm 1 / 2 5 3111¯1 , 22011 , 02011 / 2 and 0201¯1 / 2 . The variable components of the propagation vector, qx and qy , were obtained by minimising the least-squares residual between observed and calculated peak positions. The results of the indexation are given in Table 2.

Table 1 Refined cell parameters, microstrains, atomic parameters, and bond valence sums for monoclinic Tl 2 Ba 2 CuO 61d (space group F112 /m), with E.S.D. values in parentheses Temperature (K) Cell parameters ˚ a (A) ˚ b (A) ˚ c (A) g (8) ˚ 3) V (A Microstrain S/ / (%) S' (%) Atomic parameters a Tl x y z Uiso Ba z Uiso Cu Uiso O(1a) Uiso O(1b) Uiso O(2) z Uiso O(3) x y z Uiso O(4) g Uiso

˚ 2) (A ˚ 2) (A ˚ 2) (A ˚ 2) (A ˚ 2) (A ˚ 2) (A

˚ 2) (A ˚ 2) (A

296

400

500

550

600

650

700

5.4412(1) 5.5026(1) 23.1523(5) 90.127(1) 693.20(3)

5.4461(1) 5.5118(1) 23.1803(5) 90.128(1) 695.82(3)

5.4545(1) 5.5202(1) 23.2111(4) 90.122(1) 698.88(3)

5.4611(1) 5.5234(1) 23.2328(4) 90.112(1) 700.78(3)

5.4683(1) 5.5260(1) 23.2539(4) 90.098(1) 702.68(2)

5.4765(1) 5.5289(1) 23.2777(4) 90.084(1) 704.83(2)

5.4874(1) 5.5288(1) 23.3076(3) 90.034(3) 707.12(2)

0.067(5) 0.119(3)

0.065(4) 0.116(3)

0.064(4) 0.115(3)

0.060(4) 0.111(3)

0.051(4) 0.108(3)

0.048(4) 0.105(2)

0.037(3) 0.097(2)

20.0080(15) 0.0316(6) 0.20238(7) 0.0176(6) 0.08428(10) 0.0171(7) 0.0178(7) 0.0112(17) 0.0201(19) 0.11598(10) 0.0199(6) 20.0320(12) 0.0759(6) 0.28997(17) 0.0442(14) 0.075(6) 0.0442

20.0097(14) 0.0316(6) 0.20241(7) 0.0206(7) 0.08434(10) 0.0190(7) 0.0198(7) 0.0133(17) 0.0217(19) 0.11622(10) 0.0230(6) 20.0302(12) 0.0760(6) 0.28986(16) 0.0458(14) 0.076(5) 0.0458

20.0112(14) 0.0308(6) 0.20243(7) 0.0242(7) 0.08427(10) 0.0212(7) 0.0219(7) 0.0158(17) 0.0251(19) 0.11639(10) 0.0265(6) 20.0296(12) 0.0743(5) 0.28980(16) 0.0488(14) 0.080(5) 0.0488

20.0124(13) 0.0308(6) 0.20244(7) 0.0254(7) 0.08411(9) 0.0228(7) 0.0224(6) 0.0178(17) 0.0257(19) 0.11656(10) 0.0292(6) 20.0300(12) 0.0726(5) 0.28949(15) 0.0491(13) 0.078(5) 0.0491

20.0157(12) 0.0303(6) 0.20249(6) 0.0250(7) 0.08400(9) 0.0225(6) 0.0225(6) 0.0185(19) 0.0262(21) 0.11682(10) 0.0300(6) 20.0312(12) 0.0699(5) 0.28945(14) 0.0486(13) 0.068(5) 0.0486

20.0167(11) 0.0306(5) 0.20253(6) 0.0257(7) 0.08384(9) 0.0238(6) 0.0232(6) 0.0206(20) 0.0266(22) 0.11697(9) 0.0321(6) 20.0344(11) 0.0669(5) 0.28932(13) 0.0474(12) 0.064(4) 0.0474

20.0211(9) 0.0312(5) 0.20246(6) 0.0252(7) 0.08352(8) 0.0250(5) 0.0230(5) 0.0238(36) 0.0274(37) 0.11721(9) 0.0349(6) 20.0391(9) 0.0636(5) 0.28911(12) 0.0403(11) 0.054(4) 0.0403

a Atom multiplicity3occupancy and positions: Tl, 1630.5 (x, y, z); Ba, 831 (1 / 2, 0, z); Cu, 431 (0, 0, 0); O(1a), 431 (1 / 4, 3 / 4, 0); O(1b), 431 (1 / 4, 1 / 4, 0); O(2), 831 (0, 0, z); O(3), 1631 (x, y, z); O(4), 83g (1 / 4, 1 / 4, 1 / 4).

536

D.R. Pederzolli, J.P. Attfield / International Journal of Inorganic Materials 2 (2000) 533 – 541

Fig. 1. Observed, calculated and difference time-of-flight neutron diffraction profiles for monoclinic Tl 2 Ba 2 CuO 61d at 700 K from data collected on the (a) backscattering and (b) 908 detector banks of HRPD, with Bragg reflection positions for Tl 2 Ba 2 CuO 61d (lower set) and CuO (upper set) marked.

4. Discussion The temperature variation of the monoclinic angle, g, and the excess oxygen content, d 52 g(O(4)), are shown in Fig. 2. There is a clear correlation between the monoclinic angle and the excess oxygen content: g and d remain essentially constant until T¯500 K, but thereafter decrease, from g 590.122(1) and d 50.16(1) to g 5 90.034(3)8 and d 50.11(1) at 700 K. The reduction in excess oxygen content over the monoclinic-to-orthorhombic progression results in a change in electronic properties from non-superconducting to superconducting with T c 573 K. The relationship between g and d is approximately linear, and extrapolation indicates that the orthorhombic material with g 5908 would have an excess oxygen content of d ¯0.08 in Tl 2 Ba 2 CuO 61d . The thermal variation of the spacing between the Tl, Ba, O(2) and O(3) layers of atoms, and the closest copper oxide layer, is shown in Fig. 3. For each layer, this

distance increases from 296 K to ca. 500 K, due to normal thermal expansion effects. However, above ca. 500 K, the distance increases more rapidly for the Tl, O(2) and O(3) layers, but actually decreases for the Ba layer, despite the thermal expansion. These changes are shown schematically in Fig. 4 and can be rationalised in terms of charge transfer effects. As the annealing temperature is increased, interstitial oxygen O(4) is removed from within the Tl 2 O 21d bilayer in the form of oxygen gas. The excess negative charge generated by this reduction is transferred from the TlO(3) layers towards the CuO 2 planes. The enhanced negative charge in the CuO 2 planes attracts barium and repels oxygen in the BaO(2) layers. The removal of interstitial oxygen from the Tl 2 O 21d bilayer causes the latter to contract, as Tl and O(3) move closer to z51 / 4. Bond valence sums (BVS) were calculated using the formulation of Brown and Altermatt, with r 0 (Tl 31 –O 22)5 ˚ r 0 (Ba 21 –O 22)52.285 A ˚ and r 0 (Cu 21 –O 22)5 1.996 A, ˚ for B50.37 A ˚ [16,17]. The thermal variations of 1.679 A the BVS are shown in Fig. 5; the BVS at T5296 K has been subtracted from all the data for comparison. Neglecting other effects, a BVS is expected to decrease smoothly with increasing temperature, as thermal expansion increases the ion pair separations. This is well illustrated by the approximately linear temperature variations of the BVS of Ba, O(2), O(3) and O(4). However, the BVS of the in-plane oxygen atoms, O(1a) and O(1b), increase above the expected values for T .500–550 K, and show actual increases from 650 to 700 K. Also, the BVS of Cu deviates slightly below the expected value for T $550 K. These temperatures coincide with the loss of interstitial oxygen over the monoclinic-to-orthorhombic (M→O) progression (Fig. 2), which results in a change in electronic properties from overdoped and non-superconducting to near-optimally doped and superconducting with T c 573 K. The BVS deviations show the reduction in the hole concentration of the CuO 2 planes. Since the deviations of the BVS for O(1a) and O(1b) are greater than for Cu, it is likely that the holes are removed primarily from these oxygen sites, in agreement with evidence for the existence of O(2p) holes in the copper oxide planes of high-T c copper oxides [18]. The thermal variation of the microstrain parallel (S/ / ) and perpendicular (S' ) to the principal axis [001 ] is shown in Fig. 6. Since the instrumental contribution to the peak broadening from a pulsed neutron source is essentially Gaussian, the microstrains are expressed as ratios of the values at T5296 K for comparison. The microstrain in the ab-plane is almost twice that along the c-axis, showing that the orthorhombic and monoclinic macrostrains lead to microstrains through twinning or other extended defects [19]. However, S/ / shows a reduction of ca. 50% compared to a reduction of only ca. 20% in S' , over the temperature range 296–700 K. These decreases coincide with the loss of interstitial oxygen over the M→O progression. The decrease in S/ / is larger than that in S' as the strain in the c-axial direction may be caused solely by disordering of

D.R. Pederzolli, J.P. Attfield / International Journal of Inorganic Materials 2 (2000) 533 – 541

537

Table 2 Observed and calculated peak positions, and components of the propagation vector for the incommensurate structural modulation in monoclinic Tl 2 Ba 2 CuO 61d T (K)

qx

qy

hklm 1 / 2

Intensity (%)

296

0.0526

0.2205

400

0.0541

0.2205

500

0.0545

0.2196

550

0.0536

0.2195

600

0.0537

0.2193

650

0.0526

0.2189

700

0.0512

0.2181

3111¯ 1 22011 02011 / 2 0201¯ 1 / 2 3111¯ 1 22011 22011 / 2 02011 / 2 3111¯ 1 22011 02011 / 2 0201¯ 1 / 2 3111¯ 1 22011 02011 / 2 0201¯ 1 / 2 3111¯ 1 22011 02011 / 2 0201¯ 1 / 2 3111¯ 1 22011 02011 / 2 02011 / 2 22011 02011 / 2 0201¯ 1 / 2

2.3 2.6 23.2 4.9 1.9 2.5 21.8 4.6 2.0 2.3 21.3 5.1 1.7 2.5 19.9 5.1 1.3 1.8 18.1 4.7 0.7 1.5 13.9 4.3 0.3 11.1 0.7

a

˚ d obs (A)

˚ d calc (A)

1.786 1.804 2.464 3.064 1.788 1.806 2.468 3.069 1.791 1.809 2.472 3.072 1.792 1.811 2.474 3.073 1.795 1.812 2.476 3.075 1.796 1.814 2.479 3.077 1.817 2.486 3.079

1.786 1.805 2.463 3.064 1.789 1.806 2.467 3.069 1.791 1.809 2.472 3.072 1.793 1.811 2.474 3.073 1.795 1.813 2.475 3.075 1.797 1.815 2.477 3.076 1.8181 2.4778 3.0744

a

Superstructural peak intensities are calculated relative to the intensity of the g -invariant 0010 peak of Tl 2 Ba 2 CuO 61d after subtracting background ] contributions. The intensities shown for the 02011 / 2 peak include the contribution from the 204 diffraction peak.

interstitial oxygen in the Tl 2 O 21d bilayer, whereas the strain in the ab-plane may result from a combination of other effects related to the amount of interstitial oxygen, such as macroscopic monoclinic and / or orthorhombic strains, or static displacements of Tl and O(3). Static displacements of Tl and O atoms in the TlO layers of tetragonal and orthorhombic Tl-2201 have been widely

Fig. 2. Temperature variation of the monoclinic angle, g, and the excess oxygen content, d 52g(O(4)), in Tl 2 Ba 2 CuO 61d .

reported in the literature, but there is disagreement with respect to the size and direction of these displacements. For orthorhombic samples, the Tl and O(3) atoms have been refined on (0, y, z) [10,20] or (x, y, z) sites [7] under Fmmm symmetry, or (0, y, z) positions under Abma [21,22] or A2aa [17,23] symmetry. X-ray and neutron powder diffraction studies all find the y-axial displacement of O(3) to be largest [7,10,17,20–23], but there is contention as to whether it is significantly displaced along x [7,17,23], and whether Tl is displaced at all in either direction [7]. The reported magnitudes of the overall displacement from the ideal (0, 0, z) site are in the range ˚ for Tl and in the range 0.25–0.42 A ˚ for O(3). 0.14–0.19 A High-resolution neutron diffraction enables the static disorder in the TlO layers to be determined with high accuracy, by refining the Tl and O(3) atoms on general (x, y, z) positions under F112 /m symmetry. The displacements in the y-axial direction are greater than those along x, but the ratio uyu / uxu decreases from 4.0 to 1.5 for Tl and from 2.5 to 1.6 for O(3) over the temperature range 296–700 K. The magnitudes of static displacement of the ˚ for the as-prepared, Tl and O(3) atoms are 0.18 and 0.45 A non-superconducting material; these are similar to the ˚ obtained for a T c 54.5 K ‘orthovalues 0.19 and 0.42 A rhombic’ sample in a recent neutron powder diffraction study by Wagner et al. [7]. The thermal variations of the

538

D.R. Pederzolli, J.P. Attfield / International Journal of Inorganic Materials 2 (2000) 533 – 541

Fig. 4. Relative c-axial displacements of the Tl, Ba, O(2) and O(3) atoms in monoclinic Tl 2 Ba 2 CuO 61d with removal of interstitial oxygen over the monoclinic-to-orthorhombic progression giving T c 573 K, due to electronic charge transfer from TlO layers to CuO 2 planes.

in X-ray and neutron diffraction patterns [8,10] and satellite spots in electron diffraction patterns [19,24,25]. Both superstructural diffraction peaks and satellite spots become smeared out with increasing T c [8,19], suggesting that the modulation may be correlated with the interstitial oxygen content. Regions of the neutron patterns showing

Fig. 3. Temperature variation of the spacing between the Tl, Ba, O(2) and O(3) layers, and the closest copper oxide layer in monoclinic Tl 2 Ba 2 CuO 61d .

magnitude of static displacement are shown in Fig. 7. Over the range 296–700 K, the displacement of O(3) decreases ˚ to ¯0.41 A, ˚ but that of Tl increases from from ¯0.45 A ˚ to ¯0.21 A. ˚ The critical changes in these ¯0.18 A displacements coincide with the loss of interstitial oxygen over the M→O progression, for T .500 K. This suggests that the interstitial oxygen defect contributes to the local strain in the TlO layers, which is relaxed by the disordering of the Tl and O(3) atoms, and by the monoclinic and orthorhombic distortions. Several superstructural peaks due to an incommensurate structural modulation were observed in the neutron patterns. The modulation has been observed previously in orthorhombic Tl-2201, in the form of superstructural peaks

Fig. 5. Temperature variation of the change in bond valence sum, BVS (T ), BVS (T5296 K), for atoms in monoclinic Tl 2 Ba 2 CuO 61d , indicating removal of O(2p) holes in the CuO 2 planes.

D.R. Pederzolli, J.P. Attfield / International Journal of Inorganic Materials 2 (2000) 533 – 541

539

as a small number of intense qx -dependent superstructural peaks were used for its determination. It is difficult to ascertain the precise origin of the incommensurate structural modulation in monoclinic Tl2201, since its intensity may be controlled by several factors, such as interstitial oxygen content, macroscopic monoclinic strain, and static displacements of Tl and O(3), all of which vary critically over the M→O progression. The superstructural peaks in the neutron patterns are considerably more intense than have been reported in X-ray patterns [8,10], suggesting that an incommensurate ordering of oxygen atoms may be responsible for the modulation.

5. Conclusion Fig. 6. Temperature variation of the microscopic strain parallel (S/ / / S/ / (296 K)) and perpendicular (S' /S' (296 K)) to the c-axis in monoclinic Tl 2 Ba 2 CuO 61d .

the 02011 / 2 superstructural peak are displayed in Fig. 8(a), and the thermal variation of the intensities of the 02011 / 2 3111¯1 , 22011 and 0201¯1 / 2 superstructural peaks are shown in Fig. 8(b). The peak intensities start to decrease at T5500–550 K, approaching zero at 700 K, as illustrated in Fig. 8(a), where the intensity of the 02011 / 2 /204 peak combination has approximately decreased to that of the 024 peak. The refined components of the modulation propagation vector, qx ¯60.052 and qy ¯0.219, are in agreement with values reported previously: (qx , qy )5 (60.046, 0.217) [10], (60.08, 0.24) [25] and (60.07, 0.22) [8,19]. The thermal variation of qx and qy is shown in Fig. 8(c). Both components decrease for T $500 K, although the qx values may be susceptible to random error

We have reported a high-resolution high-temperature in situ time-of-flight neutron powder diffraction study of monoclinic Tl 2 Ba 2 CuO 61d , the first high-temperature neutron diffraction study reported for thallium cuprate superconductors. The amount of excess oxygen controls both the crystal structure and electronic properties. The monoclinic angle decreases with the excess oxygen content, from g 590.1278 and d 50.15 at 296 K, to g 590.0348 and d 50.11 after vacuum annealing at 700 K. This changes the electronic properties from non-superconducting to superconducting with T c 573 K, the highest found for monoclinic Tl 2 Ba 2 CuO 61d . The relative c-axial displacements of atoms in the BaO and TlO layers indicate an electronic charge transfer from the TlO layers to the CuO 2 planes. Bond valence sum calculations demonstrate that the charge transfer effects a reduction in the hole concentration of the CuO 2 planes, primarily at the in-plane oxygen sites. The thermal variation of the Lorentzian components of the diffraction peak widths indicates that the microscopic strain in the c-axial direction are caused by a disordering of interstitial oxygen in the Tl 2 O 21d bilayer, whereas the strain in the ab-plane may result from macroscopic monoclinic or orthorhombic strains, or static displacements of the Tl and O(3) atoms. Loss of interstitial oxygen results in an increase in the static displacement of ˚ but a decrease from 0.45 to 0.41 A ˚ Tl from 0.18 to 0.21 A, for O(3). An incommensurate structural modulation gives rise to superstructural peaks in the neutron patterns which decrease in intensity with the oxygen loss over the monoclinic-to-orthorhombic progression. The modulation may result from incommensurate ordering of O(4) atoms or static displacements of Tl and O(3) in the TlO layers.

Acknowledgements Fig. 7. Temperature variation of the magnitude, uDu, of the static displacement of the Tl and O(3) atoms in the TlO layers of monoclinic Tl 2 Ba 2 CuO 61d .

We thank Drs. K.S. Knight and A.J. Wright for assistance with data collection, and acknowledge the Engineer-

540

D.R. Pederzolli, J.P. Attfield / International Journal of Inorganic Materials 2 (2000) 533 – 541

Fig. 8. (a) In situ neutron diffraction patterns of monoclinic Tl 2 Ba 2 CuO 61d (T5296, 400, 500, 550, 600, 650 and 700 K), showing the intensity variation of the 02011 / 2 superstructural peak. (b) Temperature variation of the intensities of the 3111¯ 1 , 22011 , 02011 / 2 and 0201¯ 1 / 2 superstructural peaks in monoclinic Tl 2 Ba 2 CuO 61d , expressed as a percentage of the intensity of the 0010 diffraction peak. The 02011 / 2 peak intensities include a contribution ] from the 204 diffraction peak. (c) Temperature variation of the qx and qy components of the propagation vector k6qx , qy , 1l which describes the incommensurate structural modulation in monoclinic Tl 2 Ba 2 CuO 61d .

ing and Physical Sciences Research Council for the provision of beam time and for financial support for DRP.

References [1] Manako T, Kubo Y, Shimakawa Y. Phys Rev B 1992;46:11019. [2] Mackenzie AP, Julian SR, Lonzarich GG, Carrington A, Hughes SD, Liu RS, Sinclair DC. Phys Rev Lett 1993;71:1238.

[3] Manako T, Kubo Y, Shimakawa Y, Igarashi H. Phys Rev B 1991;43:7875. [4] Tsuei CC, Kirtley JR, Rupp M, Sun JZ, Gupta A, Ketchen MB, Wang CA, Ren ZF, Wang JH, Bhushan M. Science 1996;271:329. [5] Tsuei CC, Kirtley JR, Ren ZF, Wang JH, Raffy H, Li ZZ. Nature 1997;387:481. [6] Mogilevsky R, Mitchell JF, Hinks DG, Argyriou DN, Jorgensen JD. Physica C 1995;250:15. [7] Wagner JL, Chmaissem O, Jorgensen JD, Hinks DG, Radaelli PG, Hunter BA, Jensen WR. Physica C 1997;277:170. [8] Shimakawa Y. Physica C 1993;204:247.

D.R. Pederzolli, J.P. Attfield / International Journal of Inorganic Materials 2 (2000) 533 – 541 [9] Attfield JP, Aranda MAG, Sinclair DC. Physica C 1994;235:965. [10] Aranda MAG, Attfield JP, Sinclair DC, Mackenzie AP. Phys Rev B 1995;51:12747. [11] Pederzolli DR, Wltschek GM, Attfield JP. Chem Commun 1997;5:435. [12] Pederzolli DR, Wltschek GM, Fuess H, Attfield JP. Phys Rev B 1998;58:5226. [13] Rietveld HM. J Appl Crystallogr 1969;2:65. [14] Larson AC, Von Dreele RB. Los Alamos National Laboratory Report No. LA-UR-86-748, 1987. [15] Brese NE, O’Keeffe M, Ramakrishna BL, Von Dreele RB. Solid State Commun 1990;89:184. [16] Brown ID, Altermatt D. Acta Cryst B 1985;41:244. [17] Hewat AW, Bordet P, Capponi JJ, Chaillout C, Chenavas J, Godhino M, Hewat EA, Hodeau JL, Marezio M. Physica C 1988;156:369.

541

[18] Rao CNR, Raveau B. Transition metal oxides, VCH, 1995. [19] Hewat EA, Bordet P, Capponi JJ, Chaillout C, Chenavas J, Godhino M, Hewat AW, Hodeau JL, Marezio M. Physica C 1988;156:375. [20] Parise JB, Torardi CC, Subramanian MA, Gopalakrishnan J, Sleight AW, Prince E. Physica C 1989;159:239. ¨ C, Eriksson S-G, Johansson L-G, Simon A, Mattausch HJ, [21] Strom Kremer RK. J Solid State Chem 1994;109:321. [22] Parise JB, Gopalakrishnan J, Subramanian MA, Sleight AW. J Solid State Chem 1988;76:432. [23] Zetterer T, Otto HH, Lugert G, Renk KF. Z Phys B 1988;73:321. [24] Parkin SSP, Lee VY, Nazzal AI, Savoy R, Huang TC, Gorman G, Beyers R. Phys Rev B 1988;38:6531. [25] Beyers R, Parkin SSP, Lee VY, Nazzal AI, Savoy R, Gorman G, Huang TC, La Placa S. Appl Phys Lett 1988;53:432.