Structural phase transitions in pyridinium perrhenate at high pressure

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Journal of Molecular Structure 875 (2008) 58–62 www.elsevier.com/locate/molstruc

Structural phase transitions in pyridinium perrhenate at high pressure S.E. Kichanov

a,*

, D.P. Kozlenko a, J. Wasicki b, W. Nawrocik b, P. Czarnecki b, B.N. Savenko a, V.P. Glazkov c, C. Lathe d

a b

Frank Laboratory of Neutron Physics, JINR, 141980 Dubna, Moscow Region, Russia Faculty of Physics, A.Mickiewicz University, Umultowska 85, 61-614 Poznan´, Poland c RRC ‘Kurchatov Institute’, 123182, Moscow, Russia d HASYLAB am DESY, Notkestrasse 85, D-22603 Hamburg, Germany Received 26 January 2007; accepted 29 March 2007 Available online 12 April 2007

Abstract The structure of the deuterated pyridinium perrhenate (d5PyH)ReO4 (C5D5NHReO4) was studied by means of powder X-ray diffraction at high pressures up to 3.5 GPa at room temperature and neutron diffraction at high pressures up to 2.0 GPa in the temperature range 10–300 K. At ambient conditions deuterated pyridinium perrhenate has orthorhombic structure of the Cmc21 symmetry (ferroelectric phase II). At T < 250 K a phase transition to the orthorhombic phase of the Pbca symmetry (paraelectric phase III) was observed. At P > 0.7 GPa the phase transition to orthorhombic phase with the Cmcm symmetry (paraelectric phase I) was observed at ambient temperature. At P = 2 GPa the phase I is stable in the temperature range 10–293 K, indicating a suppression of the ferroelectricity in (d5PyH)ReO4 by application of high pressure. P–T phase diagram of (d5PyH)ReO4 at high pressures up to 2 GPa is discussed.  2007 Elsevier B.V. All rights reserved. Keywords: Neutron diffraction; X-ray diffraction; High pressure; Phase transition

1. Introduction Pyridinium salts belong to the group of molecular-ionic crystals with hydrogen bonds, which exhibit a reach variety of interesting phenomena such as structural phase transitions, ferroelectricity and dynamical orientational disorder of the pyridine cations [1–4]. The existence of ferroelectricity was discovered in pyridinium tetrafluoroborate (PyHBF4 – C5H5NHBF4) [1], pyridinium perchlorate PyHClO4 [2], pyridinium perrhenate PyHReO4 [3] and pyridinium periodide PyHIO4 [4]. Two latter compounds are of special interest because their Curie temperatures are above room temperature. The para-ferroelectric transition is accompanied by the structural phase transformation, resulting in the change of the disorder degree of pyridinium and perrhenate ions. The appearance of ferroelectric prop*

Corresponding author. Tel: +7 49621 63783; fax: +7 49621 65882. E-mail address: [email protected] (S.E. Kichanov).

0022-2860/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2007.03.059

erties at the phase transition is caused by the change in reorientation mechanism of pyridinium cations in such a way that the pyridinium dipoles create an average non-zero component along the ferroelectric axis. The additional deformation of tetrahedral anions may also contribute to the total spontaneous polarization [5]. In the recent study of the P–T phase diagram of PyHReO4 by dielectric and NMR measurements [6] at pressures up to 0.8 GPa and temperatures up to 350 K four phases were observed. At ambient pressure and T > 336 K PyHReO4 exhibits paraelectric phase I with the orthorhombic crystal structure of Cmcm symmetry. Below 336 K the ferroelectric orthorhombic phase II with the Cmc21 symmetry appears and at T = 250 K another structural transformation to the paraelectric orthorhombic phase III with the Pbca symmetry occurs. Under pressure, a rapid decrease of the transition temperatures to phases II and III was found. In addition, an existence of new phase II 0 with unknown structure at P > 80 MPa was observed.

S.E. Kichanov et al. / Journal of Molecular Structure 875 (2008) 58–62

The (d5PyH)ReO4 salt was prepared by allowing the d5pyridinium base (95%) dissolved in 50% ethanol to react with perrhenic acid obtained by dissolving rhenium oxide in water. The substance obtained in this way was recrystallized three times. The X-ray diffraction experiments at high pressures up to 3.5 GPa and room temperature were carried out using the multianvil X-ray system MAX80 [7] at F2.1 beamline of storage ring DORIS III. The sample was placed in the cylindrical boron nitride container with an internal diameter of 1 mm. The upper half was filled with the sample, the lower half contained sodium chloride powder for pressure calibration. The cubic boron-epoxy chamber with sample container was compressed by six tungsten carbide anvils in a large hydraulic press. Diffraction patterns were recorded in an energy dispersive mode using white synchrotron X-rays from the storage ring DORIS III. The ring operated at 4.5 GeV and a positron current of 80– 150 mA. The incident X-ray beam was collimated to 100 · 100 lm with a divergence smaller than 0.3 mrad. Spectra were recorded by a Ge solid-state detector with a resolution of 153 eV at 5.9 keV resulting in a resolution of diffraction patterns of Dd/d  1%. The Bragg angle 2h was fixed at 9.093, counting times for each diffraction pattern were about 10 min. Neutron diffraction measurements at high pressures up to 2 GPa and temperatures 10–300 K were performed with the DN-12 spectrometer [8] at the IBR-2 high flux pulsed reactor, Dubna, Russia. The sample with a volume about 2.5 mm3 was placed in the high-pressure cell with sapphire anvils [9]. The pressure was determined by measuring the shift of a ruby fluorescence line within an accuracy of 0.05 GPa. A special cryostat constructed on the basis of a closed cycle helium refrigerator was used to create a low temperature on the sample. The scattering angle was 2h = 90. The spectrometer resolution is Dd/d = 0.018 at ˚ . The typical exposition time for each pressure d=2A point was to 20 h. The refinement of powder neutron diffraction patterns were made by means of the MRIA [10] and Fullprof [11] program. 3. Results and discussion

Intensity, arb. units

2. Experimental

10000

P=3.5 GPa 5000

P=0 GPa 0

1.4

2.8

3.5

Fig. 1. The energy dispersive X-ray diffraction patterns of the (d5PyH)ReO4 measured at pressures 0 and 3.5 GPa and ambient temperature and processed by the profile matching method. Experimental points, calculated profiles and difference curve (for P = 3.5 GPa) are shown. The tick rows indicate the calculated diffraction peaks positions for high pressure phase I (upper part) and ambient pressure phase I (bottom).

evidenced. At pressures P > 0.7 GPa changes in the diffraction patterns were observed. They correspond to the appearance of the orthorhombic phase I with space group Cmcm under high pressure, as found from analysis of the data by profile matching mode refinement. The dependence of the lattice parameters of both phases are shown in Fig. 2. The linear compressibilities ki = (1/ai0) (dai/dP)T (ai = a, b, c) of unit cell parameters are ka = 0.026, kb = 0.012, kc = 0.023 GPa1 for phase II and ka = 0.010, kb = 0.0170, kc = 0.0060 GPa1 for high pressure phase I. The pressure dependence of the unit cell volume of the pyridinium perrhenate is shown at Fig. 3. The compressibility data were fitted by the third-order Birch– Murnaghan equation of state [12]:

12.4

c

12.2 12.0 8.5

a

8.0 7.5

b

7.0 6.5 0.0

X-ray diffraction patterns of deuterated pyridinium perrhenate measured at different pressures and ambient temperature are shown in Fig. 1. At ambient conditions the orthorhombic phase II with space group Cmc21 was

2.1

dhkl, Å

Lattice parameters, Å

In order to investigate the structural phase transitions as well as behavior of structural parameters of PyHReO4 in more extended pressure range than previously, we have performed combined energy dispersive X-ray diffraction and neutron diffraction measurements with a deuterated sample (d5PyH)ReO4 (C5D5NHReO4) up to 3.5 GPa.

59

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

P, GPa Fig. 2. The pressure dependence of the lattice parameters of the pyridinium perrhenate. The solid lines represent the linear fit of the experimental data.

60

S.E. Kichanov et al. / Journal of Molecular Structure 875 (2008) 58–62

observed. The unit cell parameters of both phases are presented in Table 1. The characteristic neutron diffraction patterns of the deuterated pyridinium perrhenate at the highest pressure of our study P = 2 GPa, T = 293 and 10 K are shown in Fig. 5. They correspond to the orthorhombic phase I with space group Cmcm under high pressure, as evidenced from

1.00

V/V0

0.95

0.90

0.85

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Table 1 The unit cell parameters, atomic coordinates and occupation factors of deuterated pyridinium perrhenate (d5PyH)ReO4 at different temperature and pressure

P, GPa

Atomic parameters

Fig. 3. The volume of the unit cell of the pyridinium perrhenate as a function of the pressure. The experimental data was fitted by the thirdorder Birch–Murnaghan equation of state (1).

 

3 3 P ¼ B0 x7=3  x5=3 1 þ ðB0  4Þ x2=3  1 2 4

ð1Þ

where x = V/V0 is the relative volume change, V0 is the unit cell volume at P = 0; B0 and B 0 are the bulk modulus B0 = V(dP/dV)T and its pressure derivative B 0 = (dB0/ dP)T. The calculated values B0 = 15(8) GPa, B 0 = 4(1) for the phase II and B0 = 16(3) GPa, B 0 = 4(1) for high pressure phase I are very similar. They are close to those of other pyridinium salts [6,13]. Fig. 4 shows neutron diffraction patterns of (d5PyH)ReO4 at ambient pressure and temperatures 293 and 180 K. At ambient pressure with temperature lowering at T  250 K sharp changes characteristic to the phase transition from phase II to phase III with space group Pbca were

Intensity, arb. units

2500

2000

1500

T=293 K P=0 GPa

y

Phase I T = 293 K, P = 2.0 GPa Space group Cmcm Re 0 ˚ a = 8.030(8) A O1 0 ˚ b = 6.962(7) A O2 0.180(2) ˚ c = 12.048(5) A C1/N 0.083(6)

Phase II T = 293 K, Space group Cmc21 ˚ a = 8.418(5) A ˚ b = 7.290(3) A ˚ c = 12.409(5) A

1000

2.5

3.0

3.5

4.0

dhkl, Å Fig. 4. Neutron diffraction patterns of (d5PyH)ReO4, measured at ambient pressure and T = 293 and 180 K and processed by the Rietveld method. Experimental points, calculated profiles and difference curve (for T = 180 K, bottom) are shown. The tick rows indicate the calculated diffraction peaks positions for phase II (upper part) and phase III (bottom).

0.814(3) 0.981(2) 0.662(4) 0.387(5)

C2/N

0.171(8) 0.5

D/H1

0.141(4) 0.303(9)

D/H2

0.306(7) 0.5

P = 0 GPa Re 0 O1 0 O2 0.165(7) O3 0 C1/N 0.082(9) C2/N

0.162(8)

C3/N

0.082(6)

D/H1

0.106(4)

D/H2

0.156(7)

D/H3

0.283(7)

Phase III T = 180 K, P = 0 GPa Space group Pbca Re 0.186(7) ˚ a = 16.994(5) A O1 0.089(4) ˚ b = 7.208(3) A O2 0.239(7) ˚ c = 12.084(5) A O3 0.228(8)

T=180 K P=0 GPa

2.0

x

O4 N C1 C2 C3 C4 C5 H1 D1 D2 D3 D4 D5

0.189(9) 0.145(9) 0.066(8) 0.032(8) 0.078(8) 0.158(8) 0.191(8) 0.168(7) 0.031(7) 0.051(7) 0.196(4) 0.254(7) 0.029(4)

z

Occupation 0.25 0.368(4) 0.25 0.427(8)

0.25 0.5 0.5 0.813(4)/ 0.187(4) 0.5 0.492(4)/ 0.008(4) 0.358(5) 0.830(5)/ (0.170(5)) 0.5 0.410(5)/ (0.090(5))

0.85 0 0.5 0.989(5) 0.116(4) 0.5 0.706(4) 0.015(5) 1 0.022(4) 0.097(7) 0.5 0.382(5) 0.178(8) 0.894(4)/ 0.106(4) 0.495(7) 0.246(7) 0.895(4)/ 0.105(4) 0.601(5) 0.319(8) 0.775(4)/ 0.225(4) 0.295(9) 0.110(5) 0.830(5)/ (0.170(5)) 0.689(4) 0.369(9) 0.830(5)/ (0.170(5)) 0.499(6) 0.239(5) 0.830(5)/ (0.170(5)) 0.845(7) 0.921(5) 0.930(4) 0.932(4) 0.603(4) 0.160(5) 0.169(7) 0.279(7) 0.391(7) 0.387(7) 0.271(7) 0.069(6) 0.277(4) 0.483(6) 0.479(9) 0.271(4) 0.086(9)

0.220(5) 0.231(4) 0.335(5) 0.097(7) 0.216(7) 0.072(8) 0.060(7) 0.018(7) 0.087(7) 0.075(7) 0.008(7) 0.130(5) 0.029(9) 0.150(5) 0.129(5) 0.021(9) 0.116(5)

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

S.E. Kichanov et al. / Journal of Molecular Structure 875 (2008) 58–62

and D/H in the relevant symmetry positions (Table 1). For atomic coordinates determination of the low temperature phase III we used the ‘‘rigid-molecule’’ approximation makes it possible to reduce considerably the number of independent structural parameters [11,13]. The interatomic bond lengths and bond angles for the different phases of (d5PyH)ReO4 are listed in Tables 2 and 3. The intramolecular bonds exhibit weak changes under pressure (Table 2), while intermolecular bonds are most compressible, in agreement with previous investigations [13,14]. Due to centrosymmetric character of phases I and III, they are paraelectric and ferroelectricity is observed in the phase II only [5,6]. In the disordered phases II and I

Intensity, arb. units

6000

T=293 K P=2.0 GPa

4000

T=10 K P=2.0 GPa

2000

1.5

2.0

2.5

3.0

3.5

4.0

dhkl, Å Fig. 5. Neutron diffraction patterns of (d5PyH)ReO4, measured at P = 2 GPa, T = 293 and 10 K and processed by the Rietveld method. Experimental points, calculated profiles and difference curve (for T = 10 K, bottom) are shown. The ticks below represent calculated positions of diffraction peaks (for T = 10 K).

analysis of the data by Rietveld method. No changes in diffraction patterns at P = 2 GPa on cooling down to 10 K were observed, indicating the stability of the phase I in the whole studied temperature range of 10–293 K. The structural parameters of the phase I at P = 2 GPa are presented in Table 1, their values are close to those found from previous X-ray diffraction study [5]. For the description of the structure of the disordered phases I and II, where pyridinium cations perform reorientational motion, we used averaged occupancies of the C/N Table 2 ˚ ), bond angles Selected intermolecular and intramolecular bond lengths (A () and hydrogen bond parameters of high pressure phase I of the deuterated pyridinium perrhenate (d5PyH)ReO4 at pressure P = 2 GPa and at different temperature Phase I T = 293 K, P = 2.0 GPa Re–O1 1.73(5) Re–O2 1.76(4) C1/N–C2 C1/N–D/H1 C2/N–D/H2 C1/N–C1 O1–Re–O2 O1–Re–O1 O2–Re–O2

1.33(5) 1.09(3) 1.08(5) 1.34(6) 109.2(5) 107.3(5) 112.5(3)

Phase I T = 10 K, P = 2.0 GPa Re–O1 1.77(5) Re–O2 1.72(4) C1/N–C2 C1/N–D/H1 C2/N–D/H2 C1/N–C1 O1–Re–O2 O1–Re–O1 O2–Re–O2

1.33(4) 1.07(4) 1.09(4) 1.38(5) 104.9(4) 102.5(6) 133.3 (6)

61

C1/N–O1 C1/N–O2 C2/N–O1 C2/N–O2 D/H1–O2 D/H2–O1 C1/N–D/H1–O2 C2/N–D/H2–O1

C1/N–O1 C1/N–O2 C2/N–O1 C2/N–O2 D/H1–O2 D/H2–O1 C1/N–D/H1–O2 C2/N–D/H2–O1

2.72(3) 2.85(5) 3.08(6) 3.17(9) 2.14(4) 2.23(3) 128.6(6) 117.2(6)

2.70(3) 2.95(7) 3.13(5) 3.24(6) 1.98(4) 2.28(3) 101.6(6) 140.9(6)

Table 3 ˚ ), bond angles Selected intermolecular and intramolecular bond lengths (A () and hydrogen bond parameters of the deuterated pyridinium perrhenate (d5PyH)ReO4 at ambient pressure and at different temperature Phase II T = 293 K, P = 0 GPa Re–O1 1.75(9) Re–O2 1.74(6) Re–O3 1.74(5) C1/N–C2 C1/N–C1 C2/N–C3 C1/N–D/H1 C2/N–D/H2 C3/N–D/H3 O1–Re–O2 O1–Re–O3 O2–Re–O2 O2–Re–O3 C2/N–C1–C1 C2/N–C1–D/H1 C3/N–C3–D/H3 C2/N–C3–D/H3

1.36(7) 1.38(4) 1.37(8) 1.07(3) 1.09(3) 1.09(2) 115.8(2) 99.6(4) 105.0(4) 110.8(3) 119.7(4) 117.3(4) 122.4(5) 119.2(5)

Phase III T = 180 K, P = 0 GPa Re–O1 1.74 Re–O2 1.75 Re–O3 1.77 Re–O4 1.76 N–C1 C1–C2 C2–C3 C4–C5 C5–N C1–D1 C2–D2 C3–D3 C4–D4 C5–D5 N–H

1.34 1.36 1.39 1.38 1.37 1.09 1.07 1.10 1.09 1.08 1.03

O1–Re–O2 O1–Re–O3 O2–Re–O3 O4–Re–O2 C1–N–C5 C2–C1–N C2–C1–D1 C2–C3–C4 C3–C4–D4

108.7 109.4 108.8 110.2 121.1 121.4 121.2 118.3 118.3

C1/N–O1 C1/N–O2 C2/N–O1 C2/N–O2 C3/N–O2 C3/N–O3 D/H1–O1 D/H1–O2 D/H2–O2 D/H3–O3 D/H3–O1

3.04(3) 3.43(5) 3.27(4) 3.31(5) 3.12(1) 3.02(5) 2.39(8) 2.56(8) 2.21(6) 2.39(6) 2.73(6)

C1/N–D/H1–O1 C1/N–D/H1–O2 C3/N–D/H3–O3 C3/N–D/H3–O2

117.3(7) 140.0(4) 106.2(8) 138.1(5)

H–O2 H–O4 D1–O1 D3–O4 D3–O2 D4–O2 D5–O2 D5–O3 N–O2 N–O4 C1–O1 C2–O2 C3–O4 C3–O2 C4–O2 C5–O2 C5–O3

1.81(5) 2.25(3) 2.44(6) 2.63(5) 2.82(4) 2.73(5) 2.25(4) 2.77(2) 2.35(5) 3.26(2) 3.40(3) 3.75(6) 2.88(5) 3.30(5) 3.45(6) 2.67(5) 2.82(5)

N–H–O2 N–H–O4 C1–D1–O1 C3–D3–O4 C3–D3–O2 C4–D4–O2 C5–D5–O2 C5–D5–O3

144.9(5) 168.7(5) 156.6(6) 114.4(6) 132.9(7) 118.6(8) 123.1(6) 122.0(8)

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S.E. Kichanov et al. / Journal of Molecular Structure 875 (2008) 58–62

The P–T phase diagram of deuterated pyridinium perrhenate constructed on the basis of the obtained experimental data and data from Refs. [5,6] for protonated compound are shown in Fig. 6. The stabilization of the paraelectric phase I under high pressure implies the suppression of the ferroelectricity in (d5PyH)ReO4.

350

TEMPERATURE [K]

300

I

II 250

II'

200 150

4. Conclusions

III 100 50 0 0.0

0.5

1.0

1.5

2.0

PRESSURE [GPa] Fig. 6. Phase diagram of (d5PyH)ReO4 constructed on the basis of the present data (black symbols) and data from Refs. [6,7] (open symbols).

there are weak C/N–D/H. . .O bifurcated hydrogen bonds between pyridinium cations and perrhenate anions, forming interconnected layers [5]. At ambient conditions in the disordered phase II the shortest D/H–O distances and the relevant C/N–D/H–O angles have values of 2.21 and ˚ and 117.3 and 140, respectively (Table 3). In the 2.39 A ordered phase III at ambient pressure and low tempera˚ are noticeably tures the H–O distances of 1.88 and 2.25 A shorter and N–H–O angles of 144.9 and 168.7 are larger, reflecting stronger hydrogen bonding in comparison with the phase II (Table 3). In the disordered phase I at P = 0.7 GPa and ambient temperature, the shortest D/H–O distances of 2.13 and ˚ are comparable with those for the phase II, while 2.39 A C/N–D/H–O angles of 167 and 137.4 are somewhat larger. With the pressure increase up to 2 GPa, the minimal ˚ remains nearly D/H–O distance value of 2.13 A ˚ (Table unchanged, while larger one decreases to 2.23 A 2). The relevant C/N–D/H–O angles decrease slightly to 162.9 and 134.3. The temperature variation has the opposite effect on the relative orientation of pyridinium and perrhenate ions in comparison with that of high pressure. At P = 2 GPa on cooling to 10 K a decrease of the minimal ˚ occurs, while the larger D/H–O distance value to 1.97 A distances remain nearly the same and relevant C/N–D/ H–O angles change slightly to 159.8 and 135.6 (Table 2). The thermal expansion coefficient a = 1/V(dV/dT)p for pyridinium perrhenate was calculated to be 2.98 · 104 K1 for phase I (at P = 2 GPa), 1.67 · 104 K1 for phase II and 1.38 · 104 K1 for phase III (at P = 0). The obtained values are in good agreement with the data [6].

In the present work, structural phase transitions in deuterated pyridinium perrhenate (d5PyH)ReO4 were studied by means of powder X-rays diffraction at high pressures up 3.5 GPa at room temperature and neutron diffraction at high pressures up 2.0 GPa in the temperature range 10–300 K. We found that the application of the high pressure leads to the stabilization of the paraelectric phase I, while the regions of ferroelectric phase II and paraelectric phase III are suppressed rapidly. This implies that the ferroelectricity in (d5PyH)ReO4 is unstable with respect to high pressure effects and it can be nearly fully suppressed even by moderate pressures of about 2 GPa. The variation of pressure and temperature induce different structural changes in the phase I of (d5PyH)ReO4. References [1] P. Czarnecki, W. Nawrocik, Z. Pajak, J. Wasicki, Phys. Rev. B 49 (1994) 1511. [2] P. Czarnecki, W. Nawrocik, Z. Pajak, J. Wasicki, J. Phys.: Condens. Matter 6 (1994) 4955. [3] J. Wasicki, P. Czarnecki, Z. Pajak, W. Nawrocik, W. Szczepanski, J. Chem. Phys. 107 (1997) 576. [4] Z. Pajak, P. Czarnecki, J. Wasicki, W. Nawrocik, J. Chem. Phys. 109 (1996) 6420. [5] P. Czarnecki, H. Małuszynska, J. Phys.: Condens. Matter 12 (2000) 4881–4892. [6] P. Czarnecki, A.I. Beskrovny, L. Bobrovicz-Sarga, S. Lewicki, J. Wasicki, J. Phys.: Condens. Matter 17 (2005) S3131. [7] O. Shimomura, S. Yamaoka, T. Yagi, et al., Multi-anvil type X-ray system for synchrotron radiation, in: Solid State Physics Under Pressure: Recent Advances with Anvil Devices, KTK Sci. Publ., Tokyo, 1985, pp. 351–356. [8] V.L. Aksenov, A.M. Balagurov, V.P. Glazkov, D.P. Kozlenko, I.V. Naumov, B.N. Savenko, D.V. Sheptyakov, V.A. Somenkov, et al., Physica B 265 (1999) 258. [9] V.P. Glazkov, I.N. Goncharenko, Fizika I Technika Visokih Davleniy 1 (1991) 56. [10] V.B. Zlokazov, V.V. Chernyshev, J. Appl. Crystallogr. 25 (1992) 447. [11] J. Rodriguez-Carvajal, Physica B 192 (1993) 55. [12] F.J. Birch, J. Geophys. Res. 91 (1986) 4949. [13] D.P. Kozlenko, S.E. Kichanov, J.W. Wasicki, W. Nawrocik, B.N. Savenko, V.P. Glazkov, Crystallogr. Rep. 50 (2005) 78. [14] V.P. Glazkov, D.P. Kozlenko, B.N. Savenko, V.A. Somenkov, S.Sh. Shil’shtein, Crystallogr. Rep. 44 (1999) 50.