单晶材料的制备

High Pressure Research

Vol. 24, No. 4, December 2004, pp.

481–490

PREPARATION AND SINGLE-CRYSTAL STRUCTURE

OF A NEW HIGH-PRESSURE MODIFICATION

OF BaAl 2Si 2

SHOJI YAMANAKA Ã, MASUO KAJIYAMA, SADASIVAN N. SIVAKUMAR and

HIROSHI FUKUOKA

Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University,

Higashi-Hiroshima 739-8527, Japan

A ternary element mixture of Ba, Al, and Si in a molar ratio of 1:2:2was arc-melted and treated under a high-pressure and high-temperature condition of 5GPa at 12008C. X-ray structural analysis was performed on the single crystal obtained by this treatment. The crystal was found to be a new high-pressure modiﬁcationof BaAl 2Si 2, and isotypic with layer structured ThCr 2Si 2in the space group I 4/mmm . The crystal obtained by the simple arc-melting also had the same structure (high-pressurephase). The low-pressure phase a -BaAl 2Si 2was prepared by annealing the arc-melted sample at 1200–10008C. The single crystals of the a -phase were also obtained, which crystallized in the space group Cmcm . This structure was closely related to the structure of a -BaAl 2Ge 2(spacegroup Pnma ). It is interesting to note that BaAl 2Si 2has a pressure induced polymorph, whereas BaAl 2Ge 2has a temperature dependent dimorphism.

Keywords :Silicide; High pressure; Synthesis; Clathrate; BaAl 2Si 2; Phase transition

INTRODUCTION

In a series of studies on the synthesis of new silicon clathrate compounds containing barium and iodine, we found that the use of high-pressure and high-temperature (HPHT)conditions has been effective and essential. The type I clathrate Ba 82d Si 46can be prepared under a pressure . 3GPa at 8008C [1–3].A new binary clathrate Ba 24Si 100was obtained under a pressure %1.5GPa [4].A type I clathrate containing iodine, I 8Si 44I 2, was prepared for the ﬁrsttime under a pressure of 5GPa at temperatures 800–9008C [5].The Ba containing type I silicon clathrate compounds became superconductors with a transition temperature T c ¼4–8K [3,6].This is the ﬁrstsuperconductor having a Si-sp 33D (three-dimensional)network. Note that the corresponding germanium analogs are known, such as Ba 8Ge 43

[7,8]and Ba 24Ge 100[9–11],which can be prepared under the ambient pressure by a simple melting. The iodine containing germanium clathrate, Ge 462x I 8þx (x ¼8/3), was pre-pared by the decomposition of GeI 2in Xe atmosphere [12].It is also well known that alkaline-earth metal containing Zintl disilicides MSi 2(M¼Ca, Sr, and Ba) have pressure ÃCorresponding author. Tel. /Fax:þ81-82-424-7740; E-mail:[email protected]

ISSN 0895-7959print; ISSN 1477-2299online #2004Taylor &Francis Ltd DOI:10.1080/[***********]63

482S. YAMANAKA et al .

sensitive silicon networks and undergo pressure induced structural phase transitions [13–15].Recently, it was shown that CaSi 2was transformed into MgB 2type layered structure and showed a superconducting transition of as high as 14K under a pressure of 16GPa [16].In this study, we have developed a new ternary clathrate system Ba–Al–Siand found a new high-pressure modiﬁcationof BaAl 2Si 2. The structural transition will be discussed in comparison with the germanium analog BaAl 2Ge 2, which shows a temperature-dependent structural transition [17].

EXPERIMENTAL

Materials

BaAl 2Si 2was ﬁrstprepared from the stoichiometric element mixture of Ba(99%,Katayama Chemicals), Al(99.999%,Katayama Chemicals), and Si(99.999%,Furu-uchi Chemicals) by arc-melting in a water-cooled Cu crucible in Ar atmosphere. The oxidized layer of Ba metal was removed in an Ar-ﬁlledglove box (mBraun)prior to the arc-melting. The as-melted sample was ground in the glove box and treated under HPHT conditions using a cubic multianvil type press (Riken,model CP-10). The sample was contained in an h-BN cell (5mm in inner diameter and 5mm in depth), which was placed in a carbon tube heater. The sample assembly was set in a pyrophyllite cube (20Â20Â20mm 3) as a pressure medium. The reaction temperature was monitored by a thermo-couple placed under the h-BN cell. The details of the sample assembly were shown elsewhere [2].The typical press-ing and heating modes were the following:the sample was ﬁrstcompressed to 5GPa, and heated to 12008C in 1h, and then kept for 1h at the temperature, followed by cooling to 7008C in 2h. The sample was further cooled down to room temperature in a few minutes. The applied pressure was gradually released overnight. The separate arc-melted samples were annealed in an h-BN cell vacuum-sealed in a quartz glass tube. The tube was kept in a furnace at 12008C for 1day and cooled down to 8008C for 2days, followed by cooling to room temperature for 2days.

Characterization

Powder X-ray diffraction (XRD)patterns were measured using graphite-monochromated Cu K a radiation. X-ray single crystal structural analysis was carried out using a Rigaku RAXIS diffractometer with an imaging plate (IP)area detector and graphite-monochromated Mo K a radiation. The structure was solved by direct methods and reﬁnedwith the program SHELX 97[18]and WinGX software package [19].Further crystallographic details are given in Table I. The positional, the equivalent isotropic and anisotroic displacement parameters are summarized in Table II. Differential thermal analysis (DTA)was made on the powder sample using an h-BN cell, which was sealed in a thin quartz glass tube to protect the sample from oxidation in air. The sealed sample was heated and then cooled down at a rate of 108C /min using a Shimazu DTA apparatus (TA-50WS).

RESULTS AND DISCUSSION

Arc-Melting

The powder XRD data of the arc-melted sample of the ternary mixture of BaAl 2Si 2is shown in Figure 1a. The pattern could be indexed on a tetragonal unit cell of a ¼4.199(2)and

HIGH-PRESSURE MODIFICATION OF BaAl 2Si 2

TABLE I Crystallographic data for a -and b -BaAl 2Si 2. 483

a -Phase (low-pressure)

Formula

Formula weight

Crystal size (mm)

Space group ˚) a (A˚) b (A˚) c (A˚3), Z V (A

d calcd (g/cm 3) ˚) l Mo K a (A21m (mm)

2u max (8)

Total reﬂections

Unique reﬂections

Reﬂectionswith [I ! 2s (I )]

Number of variables

GOF on F 20R 1/wR 2[I ! 2s (I )]

R 1/wR 2(alldata) BaAl 2Si 2247.460.047Â0.068Â0.13Cmcm (no.63) 4.238(5)10.890(5)10.106(5)466.4(6),43.5240.710699.20054.[1**********]181.1690.0266/0.04630.0305/0.0467b -Phase (high-pressure)BaAl 2Si 2247.460.16Â0.14Â0.025I 4/mmm (no.139) 4.231(2)4.231(2)12.601(9)225.6(4),23.6440.710699.51254.[1**********]91.1270.0154/0.03670.0155/0.0367

˚. This crystal was different from the BaAl 2Si 2prepared by Carrillo-Cabrera c ¼12.873(6)A

et al. [20]using a glassy carbon crucible. This crystal is presumably isotypic with the ThCr 2Si 2structure [21],a new modiﬁcationof BaAl 2Si 2.

HPHT Treatment

The arc-melted sample was treated in the h-BN cell under HPHT conditions. Figure 1c shows the XRD pattern of the sample obtained by the treatment at 5GPa and 12008C. The detailed condition was described in Experimental section. The XRD pattern of the HPHT-treated sample could be indexed on a similar tetragonal cell with a ¼4.231(5)and ˚. Although the lattice constant c was slightly decreased from 12.873A ˚of c ¼12.601(5)A

the as-arc-melted sample, it was apparent that the structure was essentially unchanged by the HPHT treatment.

Annealing the Crystals

The HPHT-treated samples were annealed in h-BN cells vacuum-sealed in quartz glass tube at 800–12008C. Figure 1b shows the XRD pattern of the sample obtained after annealing.

˚2) for a -and TABLE II Atomic coordinates, equivalent isotropic and anisotropic temperature factors (A

b -BaAl 2Si 2.

Site x y z U 11U 22U 33U 23U 13U 12

0000000U eq 0.0128(2)0.0285(7)0.0343(6)0.0239(3)0.0236(5)0.0248(5)a -BaAl 2Si 2(Low-pressurephase, space group Cmcm ) Ba 4c 00.0776(6)0.25000.0128(3)0.0097(3)0.0159(3)0Al 8f 00.3888(3)0.0326(2)0.0073(9)0.059(2)0.0193(11)20.0239(11)Si 8f 00.7942(1)0.1244(3)0.0137(10)0.0106(11)0.0786(19)20.0101(11)b -BaAl 2Si 2(High-pressurephase, space group I 4/mmm ) Ba 2a 0000.0216(3)0.0216(3)Al 4d 1/201/40.0158(6)0.0158(6)Si 4e 1/21/20.1361(2)0.0160(6)0.0160(6)0.0286(5)0.0391(13)0.0424(13)000

Note :U eq is deﬁnedas one-third of the trace of the orthogonalized U tensor.

484S. YAMANAKA et al .

FIGURE 1Powder XRD patterns of BaAl 2Si 2prepared in different conditions:(a)arc-melted, (b)annealed at 12008C, and (c)HPHT treated.

The pattern was changed to that of an orthorhombic symmetry of a ¼4.238(5),˚. Single crystals were obtained by annealing from a b ¼10.890(5),and c ¼10.106(5)A

temperature of 12008C and were used for the structural analysis.

DTA Analysis

Figure 2shows the DTA curves measured on the crystals obtained by the HPHT treatment and the annealing at 12008C. The annealed sample shows a sharp endothermic peak due to the congruent melting of the sample at 10278C and an exothermic peak at 10178C due to the crystallization on the cooling process. The HPHT sample also shows similar sharp endother-mic and exothermic peaks coupled with a broad endothermic peak starting from about 5008C. After melting, the HPHT-treated sample did not show the broad endothermic peak in the second measurement.

Single Crystal Analysis

The two types of single crystals, the HPHT-treated sample (high-pressurephase or b -BaAl 2Si 2) and the annealed sample (low-pressurephase or a -BaAl 2Si 2) were performed, and the results are shown in Tables I and II. The high-pressure phase crystallized in the space group I 4/mmm , isotypic with the popular crystal system of the ThCr 2Si 2structure [21].This is isotypic with the high-temperture phase of b -BaAl 2Ge 2reported by Leoni et al. [17].The annealed or low-pressure phase crystallized in the space group Cmcm . This is isotypic with the low-temperature phase of a -BaAl 2Ge 2(spacegroup Pnma ). The space group Cmcm for a -BaAl 2Si 2is a minimal non-isomorphic supergroup of Pnma for a -BaAl 2Ge 2[22].The Si and Al sites cannot be distinguished by x-ray. The Si and Al sites of BaAl 2Si

HIGH-PRESSURE MODIFICATION OF BaAl 2Si 2485

FIGURE 2DTA curves of (a)annealed and (b)HPHT-treated samples.

were tentatively assigned to the Ge and Al sites of BaAl 2Ge 2, respectively. Some typical interatomic lengths and bond angles are given in Table III.

Structural Details and Transitions

The germanide analog BaAl 2Ge 2has been studied by Leoni et al. [17].They found that the germanide shows a temperature dependent dimorphism; the 3D network of the low-temperature a -phase (thea -BaCu 2Si 2structure [23],space group Pnma ) transforms into the layer structured high-temperature b -phase (theThCr 2Si 2structure, space group I 4/mmm ). The transformation occurs reversibly at 1100K, and the crystal melts incongruently at 1310K. Similar dimorphisms were found in BaCu 2S 2and BaCu 2Se 2

[23,24].It is interesting to note that the b -phase of BaAl 2Si 2in this study was obtained as a high-pressure phase, which is transformed into the a -phase by annealing at

elevated

486S. YAMANAKA et al .

TABLE III Main interatomic distances d and bond angles for a -and b -BaAl 2Si 2.

a -BaAl 2Si 2

Distances

Ba 222Si

Ba 224Si

Ba 224Al

Ba 222Al

Si 22Si

Al 22Al

Si 22Al

Angles (8)

Al 22Si 22Al

Si 22Al 22Si ˚) d (A3.318(2)3.430(3)4.040(2)3.577(3)3.669(2)4.056(2)2.538(4)2.510(3)2.532(2)2.548(3)113.61(10)95.15(9)113.61(10)

84.85(9)

119.24(7)

126.28(11)Distances Ba 228Si Ba 228Al Ba 222Si b -BaAl 2Si 2˚) d (A3.448(3)3.795(1)4.591(1)Si 22Si Al 22Al Si 22Al Angles (8) Al 22Si 22Al Si 22Al 22Si 3.433(3)2.991(1)2.557(1)111.67(9)71.62(4)111.67(6)108.38(3)

temperatures. It is reasonable to consider that the a -phase of BaAl 2Si 2should be characterized as a low-pressure stable phase, i.e. , the dimorphism of BaAl 2Si 2is pressure dependent. Note that the high-pressure phase of BaAl 2Si 2was also obtained by arc-melting in the ambient pressure. This can be interpreted as follows:in the arc-melting, the melted sample was rapidly cooled and quenched to room temperature, and the surface of the melt was ﬁrstsolidiﬁedinto a hard crust. The melt inside the crust will be then cooled. If the sample volume of the melt increases upon the crystallization, it is subjected to a high pressure in the conﬁnementof the crust, resulting in the formation of the high-pressure phase.

As can be seen from Table I, the theoretical density of b -BaAl 2Si 2is larger than that of the a -phase by about 5%and the b -phase should be favorable under high pressure. On the con-trary, the densities of a -and b -BaAl 2Ge 2are calculated to be 4.66and 4.74g /cm 3, respect-ively. The difference between the two phases of BaAl 2Ge 2is much smaller, and thus the higher symmetrical b -phase will be favored at high temperatures. A very broad endothermic peak was observed in the DTA curve of the HPHT sample of Figure 2. This curve suggests that the transition of the b -phase to the a -phase starts very gradually from about 5008C on heating in the ambient pressure.

Leoni et al. calculated the band structure of BaAl 2Ge 2and showed that the electron localization analyzed by electron localization function (ELF)supported the Zintl-Klemm scheme; the valence electrons of Ba are completely transferred to the network, and the 0bonding can be written in the form of Ba 2þ[Al2Ge 2]22or Ba 2þ[Al22Ge 2].

The crystal structures of b -and a -BaAl 2Si 2are schematically shown in Figures 3and 4, respectively. In the structure of the high-pressure b -phase, the Si–Sidistance between the ˚, which is much larger than the bond distance Si 2[Al2Si 2]22layers is 3.433(3)A 2Al ˚) within the layers. This reﬂectsthe layered character of the b -phase. The Ba (2.559(1)A

atoms are located between the [Al2Si 2]22layers. Leoni et al. [17]estimated a possible phase transition route for the reconstructive structural changes between the a $b phases of BaAl 2Ge 2. They used topological descriptors, Periodic Nodal Surfaces (PNS),and explained how the 3D a -BaAl 2Ge 2was transformed into the layer structured (2D)b -BaAl 2Ge 2on heating. In case of annealing of the high-pressure phase b -BaAl 2Ge 2

HIGH-PRESSURE MODIFICATION OF BaAl 2Si 2487

FIGURE 3Structure of b -BaAl 2Si 2(a)and the coordination to a Ba atom (b).

of this study, the direction is reversed. A part of the Si 22Al bonds of puckered [Al2Si 2]layers will open, and the layers are interconnected into the 3D network of a -BaAl 2Si 2. Unlike the temperature dependent dimorphism of BaAl 2Ge 2, this is not a reversible transition. The Ba atom in the a -BaAl 2Si 2is located in a polyhedron formed by two 8-membered rings, three 6-membered rings, and four 4-membered rings (826344) as shown in Figure 4(b),and surrounded by 18network atoms (@Si8Al 10). Such polyhedra share the faces to form the 3D network. The two Si 22Si and two Al 22Al bond pairs are alternately connected to form bent 8-membered rings of the polyhedron. The bond length of the Si 22Si bonds formed ˚, which is the averaged between the [Al2Si 2]layers by the structural transition is 2.542(4)A

value of the Al 22Al and Si 22Al bond lengths of the [Al2Si 2]network of the a -BaAl 2Si 2. The structure of the a -phase is three dimensional.

The low-temperature a -BaCu 2S 2, a -BaCu 2Se 2, and, a -BaAl 2Ge 2phases are isostructural and all crystallize in the space group Pnma ; the two kinds of network atoms are distinguish-able, and occupy different sites as shown in Figure 5for a -BaAl 2Ge 2[17].In case of a -BaAl 2Si 2, however, the Al and Si atoms cannot be distinguished by x-ray, and the compound crystallizes in the space group Cmcm , a minimal non-isomorphic supergroup of Pnma . A similar situation occurs in the structure of EuGa 2Ge 4, where Ga and Ge atoms cannot be distinguished. EuGa 2Ge 4crystallizes in the space group Cmcm , and the E(1/3Ga þ2/3Ge) atoms form a similar polyhedral network found in a -BaAl 2Si 2[20].The two structures of a -BaAl 2Si 2and EuGa 2Ge 4are compared in Figure 6. Carrillo-Cabrera et al . [20]proposed that the structure of EuGa 2Ge 4can be seen as an intergrowth structure with a -BaAl 2Si 2layers as segments coupled with thin puckered layers composed of 5-and 6-membered rings. They also proposed that EuGa 2Ge 4can be viewed as a new clathrate structure. Although we have tentatively assumed that the Si and Al positions correspond to those of Ge and Al in BaAl 2Ge 2, it is also very likely that Si and Al sites are disordered. This must be solved by further

studies.

488S. YAMANAKA et al .

FIGURE 4Structure of a -BaAl 2Si 2(a)and the coordination polyhedron (@Si8Al 10) containing a Ba atom (b).CONCLUSIONS

Barium containing Si-rich compounds such as Ba 82d Si 46and Ba 24Si 100are prepared only by using HPHT conditions. The silicon atoms form 3D polyhedral networks, the Ba atoms being trapped in the polyhedra. It is interesting to compare these compounds with the corresponding germanide analogs, which can be prepared in the ambient pressure by simple melting. In this study, we have found another isomorphous silicide–germanidepair, BaAl 2Si 2and BaAl 2Ge 2, where high-pressure conditions favor the formation of

the

HIGH-PRESSURE MODIFICATION OF BaAl 2Si 2489

FIGURE 5Structure of a -BaAl 2Ge 2

[17].

FIGURE 6Comparison of the structures of (a)a -BaAl 2Si 2and (b)EuE 6(E¼1/3Ga þ2/3Ge), and (c)The atomic arrangement of the shaded polyhedra (Ba@Si6Al 8) of a -BaAli 2Si 2.

silicide. The high-pressure phase b -BaAl 2Si 2is isomorphous with the high-temperature phase b -BaAl 2Ge 2. It is also noteworthy that the same high-pressure phase of b -BaAl 2Si 2can be obtained by the arc-melting in the ambient pressure. This is probably due to the rapid cooling condition of the arc-melting

method.

490S. YAMANAKA et al .

Acknowledgements

This study has been supported by a Grant-in-Aid for ScientiﬁcResearch (A)(no.16205027) and the COE Research (no.13E2002) of the Ministry of Education, Culture, Sports, Science and Technology of Japan, and Electric Technology Research Foundation of Chugoku. References

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