ThCr2Si2-type compounds

Data concerning the magnetic properties of the ThCr2Si2 type compounds with the chromium are scarce. 

In the review by Kanatzidis et al. (2005), the preparation by the tin-flux method is mentioned also for several ternary phosphides and polyphosphides of rare-earth and transition metals. Typically the components (R metal, T metal, P and Sn in an atomic ratio of about 1 4 20 50) in sealed silica tubes were slowly heated, to avoid violent reactions, up to 800°C, annealed at that temperature for 1 week and slowly (2 K/h) cooled to ambient temperature. The tin-rich matrix was dissolved in diluted hydrochloric acid. The authors described the preparation of compounds corresponding for instance to the formula MeT4P12 (Me = heavy rare-earth metals and Th and U, T = Fe, Ru, etc.) and to the series of phases MeT2P2 (Me is a lanthanide or an actinide and T a late transition metal) having a structure related to the BaAl4 or ThCr2Si2 types. 

ThCr Si2, tllO structural-type (a BaAl -derivative type) The ThCr2Si2-type structure is one of the ordered ternary variant of the BaAl4 type, frequently found in several ternary compounds. 

The ThCr2Si2 More than 400 compounds of AB2X2 stoichiometry adopt the ThCr2Si2 type struc-... 

Eu-Ni-Sb. Marchand and Jeitschko (1978) investigated the crystal structure of the EuNi2Sb2 compound (ThCr2Si2 type structure a = 0.43834, c = 1.0664 X-ray powder diffraction). Stoichiometric amounts of the elemental components (Eu, Ni better than 99.5%, Sb better than 99%) were sealed under argon in silica tubes, heated at 920 K for 12 h, followed by 4 h at 1250 K and rapid cooling. The samples were then ground to powder, cold pressed to... 

Only two ternary compounds have been characterized in the Gd-Mn-Si system. GdMn2Si2 adopts the ThCr2Si2-type of structure I4/mmm, a = 3.950, c = 10.478, = 6.57 kg/dm (Rossi et al., 1978 X-ray powder diffraction). Samples... 

According to an X-ray powder analysis by Mayer et al. (1972) the compound SmAg2Si2 crystallizes with the ordered ThCr2Si2-type of structure [14/mmm, a = 4.169(5) and c= 10.69(5)]. For sample preparation, see LaAg2Si2. Alloys were multiphase also containing some excess Ag as well as small amounts of SmSij phases (tetragonal and/or orthorhombic modification ). 

The ordered ThCr2Si2-type of structure was recently confirmed by Nguyen et al. (1983) for the compound TbNi2Si2 (from neutron diffraction data at room temperature 0 = 3.952(3), c = 9.542(2), z(Si) = 0.372). Samples were prepared from 4N materials by melting in an induction furnace followed by a heat treatment at 900° C... 

All lanthanide compound series exhibit the lanthanide contraction when one considers the molar volume of the compound. But in four of the compound series which have the tetragonal ThCr2Si2-type structure the c lattice parameter actually expands, but a decreases sufficiently such that the unit-cell volume shows the normal lanthanide contraction (Gschneidner 1986). These compound series are RM2Si2, with M = Cu, Ag and Au, and RCu2Ge2- Although this lanthanide expansion was originally observed over 15 years ago, no one has proposed an explanation. 

Most compounds have the body-centered tetragonal ThCr2Si2 type structure (Ban and Sikirica 1964). Some exhibit the primitive tetragonal CaBe2Gc2-type structure (Eisenmann et al. 1972). 

The crystal structure of ThCr2Si2 type is characterized by two parameters the free parameter z and the ale ratio. The values of these parameters for rare-earth compounds change with the kind of transition metal (Szytula and Leciejewicz 1987a). The z values oscillate around 0.375 value. For compounds containing Pd and Cu z> 0.375, in contrast to compounds containing Ru and Os for which z< 0.375. The ale values observed are in the range from 0.366 to 0.424. The values of these parameters indicate that the tetrahedra deviate from the regular shape. 

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