Physics Letters A 205 (1995) 77-80
Phase transition of ZrRuP at high temperatures and high pressures Ichirnin Shirotani
a, Kenji Tachi a, Nobuharu Ichihashi a, Takafumi Adachi b, Takumi Kikegawa b, Osamu Shimomura b
* Muroran Institute of Technology, 27-1, Mizumoto, Muroran-shi 050, Japan b Photon Factory, National Laboratory for High Energy Physics, Oho, Tsukuba-shi 305, Japan Received 30 June 1995; accepted for publication Communicated by L.J. Sham
8 July 1995
Abstract Using synchrotron radiation X-ray diffraction ZrRuP was studied at high temperatures and high pressures. The interesting superconductor ZrRuP crystallizes in two modifications: the Fe,P-type hexagonal structure (h-ZrRuP) has T, of 13 K and the Co,P-type orthorhombic form (o-ZrRuP) has T, of about 4 K. Both phosphides are layer compounds. Each layer in h-ZrRuP is occupied by either Zr and P atoms or Ru and P atoms. In contrast, the layers of o-ZrRuP are filled with Zr, Ru and P atoms. The compressibility of o-ZrRuP was measured up to 4 GPa at room temperature and thermal expansion was studied from room temperature to 1000°C at around 3.5 GPa. Further, we have observed in situ that the orthorhombic phase of ZrRuP transforms to the hexagonal one at around 1100°C under 3.5 GPa. The phase transition of the phosphide is discussed.
1. Introduction The interesting superconductor ZrRuP crystallizes in two modifications; the Fe,P-type hexagonal structure (h-ZrRuP) has T, of 13 K [1,2] and the Co,Ptype orthorhombic form (o-ZrRuP) has T, of about 4 K [2,3]. Both metal phosphides are layer compounds. Each layer in the hexagonal lattice of h-ZrRuP is occupied by either Zr and P or Ru and P atoms. The two-dimensional triangular clusters of Ru, are formed and linked with each other through Ru-P bonds in the basal plane. In contrast, the layers of o-ZrRuP are filled with Zr, Ru and P atoms. These layers are all equivalent. We have prepared ZrRuP at high temperatures and high pressures 121. The temperature of preparation of o-ZrRuP is lower than that of h-ZrRuP at high pressure . The density of ZrRuP is 8.172 0375-9601/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0375-9601(95)00526-9
g/cm3 for the orthorhombic phase and 8.148 g/cm3 for the hexagonal phase at room temperature and atmospheric pressure [1,3]. The density of o-ZrRuP is slightly larger than that of h-ZrRuP. These results suggest that the hexagonal form is a high temperature phase of ZrRuP. Using synchrotron radiation we have studied X-ray diffraction of ZrRuP at high temperatures and high pressures, and observed in situ that the orthorhombic phase transforms to the hexagonal one at around 1100°C under 3.5 GPa.
2. Experimental A cubic-anvil type high pressure apparatus was used for the X-ray study with synchrotron radiation at high temperatures and high pressures. The sample container made of a mixture of boron and epoxy
I. Shirotani et al. /Physics
Letters A 205 (1995) 77-80 -I
Fig. 1. Sample assembly of X-ray diffraction at high temperatures and high pressures. (1) Pyrophyllite disk; (2) sample; (3) thermocouple; (4) carbon furnace; (5) pyrophyllite cylinder; (6) pyrophyllite plug; (7) BN disk; (8) BN sleeve; (9) BN disk; (10) NaCl used as the pressure sensor; (11) carbon disk.
resin is formed into a cube of 8 mm on an edge. Fig. 1 shows a sample assembly for the measurement of X-ray diffraction at high temperatures and high pressures. The diffraction patterns of ZrRuP were recorded at 28 = 4” and 6” with an energy dispersive method. The diffraction lines of NaCl were used to determine the pressure value according to Decker’s scale. Metal phosphides were prepared with a wedgetype cubic-anvil high pressure apparatus . The sample assembly for the preparation of metal phosphides is similar to that used for the synthesis of black phosphorus . o-ZrRuP was synthesized by the reaction of stoichiometric amounts of Zr, Ru and red phosphorus powders at 1000°C and 4 GPa.
3. Results and discussion The crystal data of o-ZrRuP are given in Table 1. The c-axis is perpendicular to the layers Cab plane).
Table 1 Crvstal data. comuressibilitv
and thermal exoansion
Temperature Fig. 2. Lattice constants and cell volume temperatures and 3.5 GPa.
a, Present work Compressibility (10-3/GPa) Thermal expansion at = 3.5 GPa (10w6rC)
7.3215 7.3200 - 2.01 9.26
3.8623 3.8680 - 2.51 3.90
181.46 181.24 -6.11 23.8
6.4012 - 1.64 10.3
The lattice constants (a,,, b, and co> and the cell volume (V,) at atmospheric pressure decrease linearly with increasing pressure up to 4 GPa at room temperature. The compressibilities of o-ZrRuP are summarized in Table 1. The c-axis shrinks easily compared with the a- and b-axes. The cell volume at 4 GPa reduced to 97.5% of V, at room temperature. The X-ray diffraction of o-ZrRuP was studied from room temperature to 1300°C at around 3.5 GPa. Fig. 2 shows the temperature dependence of the lattice parameters and the cell volume of o-ZrRuP at around
I. Shirotani et al. /Physics
Letters A 205 (1995) 77-80
h-ZrRuP OGPa 25’C
ENERG7t (keGW Fig. 3. Energy dispersive
0 l .
.0’ lo .O- .O* 0 -&eo
a=6.4169A b=7.3215A c-3.8623A
0 RU : z=1/4 0 P :2=1/4 0 zr : 2=3/4 0 Ru : z=3/4 l P : z=3/4
Fig. 4. Structures
and 3.5 GPa.
1000°C. The thermal expansion coefficients of o-ZrRuP are given in Table 1. The c-axis is sensitive to pressure but insensitive to temperature. The a- and
profiles of (a) o-ZrRuP and (b) h-ZrRuP at high temperatures
3.5 GPa. The lattice constants (a,, b,,, caT) and the volume (V,,) at room temperature and 3.5 GPa increase linearly with increasing temperature up to
of the a6 plane of ZrRuP: (a) orthorhombic
form; (b) hexagonal
I. Shirorani et al. /Physics
b-axes considerably expand with increasing temperature. Fig. 3a and 3b show the energy dispersive X-ray diffraction pattern of o-ZrRuP at high temperatures and 3.5 GPa. The diffraction pattern of o-ZrRuP was observed at around 600°C and 3.5 GPa. The orthorhombic phase began to be transformed into the hexagonal one at around 1100°C and 3.5 GPa. Both phases coexisted between 11OO’C and 1300°C. A single phase of h-ZrRuP was observed at around 1300°C under 3.5 GPa. This result indicates that the hexagonal phase is the high temperature phase of ZrRuP. The diffraction pattern of h-ZrRuP was also found when the temperature and pressure were reduced to room temperature and atmospheric pressure. h-ZrRuP was quenched and was very stable at the normal condition. Fig. 4 shows the projections of o- and h-ZrRuP onto the ab plane. Both phosphides are layer compounds. All atoms have positions in the layers parallel to the ab plane and separated by a distance of ic. Each layer in h-ZrRuP is occupied by either Zr and P or Ru and P atoms. The two-dimensional triangular clusters of Ru, are formed in the ab plane. The layers of o-ZrRuP are filled with Zr, Ru and P atoms. The cluster of Ru, is not formed in o-ZrRuP. The only similarity to the hexagonal phase is that zirconium chains still exist along the a-axis (x-direction). As is indicated by the dotted line, large hexagons of P atoms exist in the ab plane of h-ZrRuP. We can see similar hexagons in the orthorhombit phase though the height of the P atoms is different. There are three Zr and Ru atoms in the large hexagons formed by P atoms for both phosphides. A hexagon of h-ZrRuP almost seems to be superposed
Letters A 205 (1995) 77-80
on a hexagon of o-ZrRuP. However, the hexagons of o-ZrRuP are slightly deformed, and their area is slightly smaller than that of h-ZrRuP. As shown in Table 1, the a- and b-axes of o-ZrRuP easily expand with increasing temperature. The area of the hexagons in o-ZrRuP is close to that in h-ZrRuP at higher temperatures. The deformed hexagons in o-ZrRuP transform to the regular hexagons in h-ZrRuP at around the transition temperature. h-ZrRuP which is the high temperature phase of ZrRuP is quenched and behaves as an interesting superconductor at the normal condition. This phosphide has a large upper critical field (If,,) of 17.5 T at 0 K . We have found the new superconductor ZrRuSi ; this metal silicide has two modifications, the Fe, P-type hexagonal structure and the Co, P-type orthorhombic one. The T, are about 12 K for h-ZrRuSi and 5 K for o-ZrRuSi. A similar phase transition is also expected for ZrRuSi.
References [l] H. Barz, H.C. Ku, G.P. Meisner, Z. Fisk and B.T. Mattias, Proc. Natl. Acad. Sci. 77 (1980) 3132.  I. Shirotani, N. Ichihashi, K. Nozawa, M. Kinoshita, T. Yagi, K. Suzuki and T. Enoki, Japan. J. Appl. Phys. Suppl. 32-3 (1993) 695.  R. Muller, R.N. Shelton, J.W. Richardson and R.A. Jacobson, J. Less. Common Met. 92 (1983) 177. (4) 0. Shimomura, S. Yamaoka, T. Yagi, M. Wakatsuki, K. Tsuji, 0. Fukunaga, H. Kawamura, K. Aoki and S. Akimoto, Mat. Res. Sot. Symp. Proc. 22 (1984) 17.  I. Shirotani, Mol. Cryst. Liq. Cryst. 86 (1982) 1943; I. Shirotani, S. Shiba, K. Takemura, 0. Shimomura and T. Yagi, Physica B 190 (1993) 1191.  1. Shirotani, K. Tachi, K. Takeda, S. Todo, T. Yagi and K. Kanoda, submitted.