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First-order phase transitions with strong and extremely anisotropic changes of the lattice parameters were observed in the ThCr 2 Si 2 structure-type compounds EuCo 2 P 2 and SrNi 2 P 2. At room temperature, with increasing pressure the phase transition occurs in SrNi 2 P 2 at 4 kbar and in EuCo 2 P 2 at 30 kbar which is in the latter probably accompanied by a valence change of Eu. On the basis of single-crystal data of ACo 2 P 2 (A ϭ Ca, Sr, La, Ce, Pr, Nd, Eu͒ at ambient pressure and temperature we discuss the pressure dependence of the bond lengths in these compounds. ͓S0163-1829͑97͒00346-9͔

ARh 2 As 2 (A: Eu, Sr, Ba) and BaZn 2 As 2 . -Single crystals of the title compounds are grown from the elements in a Pb or Pb/Bi flux at 1100 °C. EuRh2As2 and BaRh2As2 crystallize with the well-known ThCr2Si2 type structure (tetragonal, space group I4/mmm, Z = 2). Due to the rigid layers of RhAs4 tetrahedra and to the atomic size of Eu and Ba, the As-As distances between the layers are very long (2.97 and 3.66 Å, resp.). SrRh 2 As 2 is polymorphic and undergoes two phase transitions at about 190 and 282 °C. Main features of the three crystal structures are also layers of RhAs4 tetrahedra. At room temperature α-SrRh2As2 probably crystallizes with the BaNi2Si2-type (space group Cmcm, Z = 4), while β-SrRh2As2 (space group Fmmm, Z = 4) forms a new orthorhombically distorted variant of the ThCr2Si2-type. γ-SrRh2As2 crystallizes with the ThCr 2 Si 2 type structure. The same is true for the high temperature modification of BaZn2As2 (β-phase), whereas the already known α-BaZn2As2 phase exhibits the α-BaCu 2 S 2 type structure (space group Pnma, Z = 4) consisting of a three-dimensional-network of edge-and corner-sharing ZnAs4 tetrahedra wit Ba atoms in the voids of this network. -(HELLMANN, A.; LOEHKEN, A.; WURTH, A.; MEWIS*, A.; Z. Naturforsch., B: Chem. Sci. 62 (2007) 2, 155-161; Inst. Anorg. Chem. Strukturchem., Heinrich-Heine-Univ., D-40225 Duesseldorf, Germany; Ger., Abstr. Eng.) -Schramke 18-007

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