Modulated crystal structures and phase transitions - The compounds SrPt2As2 and EuPt2As2

The new compounds SrPt2As2 (a = 4.482(1), b = 4.525(1), c = 9.869(2) angstrom) and EuPt2As2 (a = 4.435(2), b = 4.467(2), c = 9.784(3) angstrom) have been prepared by reaction of the elements at 1050 degrees C. Satellite reflections observed in X-ray diffraction experiments on single crystals revealed that both compounds have modulated crystal structures at room temperature. The average structures can be derived from the CaBe2Ge2 type and described in the orthorhombic space group Pnimn (Z = 2) with splitted positions for half the Pt and As atoms. Based on the average structure of SrPt2As2, the modulated structure has been solved and refined using the (3+ I)-dimensional superspace group Pmmn(alpha 00)0s0 and a modulation vector q = 0,62 center dot a*. The modulation appears in the layers of the PtAs4 tetrahedra with occupational and displacive components. The other atoms are also slightly affected by the modulation. With increasing pressure, SrPt2As2 undergoes a phase transition at about 60 kbar. The crystal structure of the high pressure phase (HP-SrPt2As2: a = 9.752(2), b = 4.398(1), c = 8.727(2) angstrom, beta = 90.34(3)degrees; 66(5) kbar) is a monoclinic variant (P2(1)/c; Z = 4) of the tetragonal CaBe2Ge2 type known from EuPt2Ge2. Band structure calculations reveal that a Peierls-distortion within the Pt square nets is a reason for the structural instabilities in SrPt2As2. The satellite reflections of EuPt2As2 become weaker with increasing temperature and disappear above 200 degrees C. The high temperature phase (HT-EUPt2As2: a = 4.464(1), c = 9.832(1) angstrom; 350 degrees C crystallizes with the CaBe2Ge2 type (P4lnmm; Z = 2). High thermal displacement parameters of the Pt and As atoms of the PtAs4 tetrahedra cover the effects of the modulation at room temperature. Several analogous compounds were studied by X-ray diffraction resulting in the following lattice parameters: a = 4.5640(1), c = 10.021(2) angstrom for BaPt2As2, a = 4.603(2), c = 10.565(2) angstrom for SrPt2Sb2, a = 4.630(2), b 4.683(2), c = 10.389(3) angstrom for EuPt2Sb2, and a = 4.458(1), c 9.823(2) A for SrIr2As2.