SrNi10P6, EuNi10P6, and BaCo10As6: Phase transitions and crystal structures

SrNi10P6, EuNi10P6 and BaCo10As6 were prepared by heating mixtures of the e lements in the range of 800 degrees-1000 degrees C and were investigated by means of single-crystal Xray methods. At higher temperatures the isotypic Ni phosphides (HT-SrNi10P6: a = 6.481(2), b = 16.080(4), c = 8.763(2) Angst rom (350 degrees C); HT-EuNi10P6: a = 6.509(2), b = 16.063(4), c = 8.766(4) Angstrom (500 degrees C)) crystallize in the BaNi10P6 type structure (Cmca ; Z = 4), which can be described as an arrangement of Ni14P12 cages with Sr or Eu atoms in the centres. The cages are linked to layers separated by ad ditional Ni atoms, which are coordinated tetrahedrally by P atoms of differ ent cages. Cooling down both compounds undergo from about 270 degrees C (Sr Ni10P6) and 410 degrees C (EuNi10P6) respectively a second-order phase tran sition involved with a change to an orthorhombic P lattice. In the structur e of the NT phases (Pnma; Z = 4; NT-SrNi10P6: a = 15.993(1), b = 6.473(1), c = 8.735(1) Angstrom; NT-EuNi10P6: a = 15.925(1), b = 6.478(1), c = 8.720( 1) Angstrom (25 degrees C)) the Ni14P12 cages are slightly distorted in com parison with the high temperature modifications. BaCo12As6 (a = 16.405(9), b = 6.858(4), c = 8.955(7) Angstrom) crystallizes in the same structure (Pn ma), but doesn't exhibit a comparable phase transition up to 600 degrees C. Measurements of the susceptibility of EuNi10P6 between 4 K and 850 K showed divalent Europium and no magnetic order down to 4 K.