Structure and properties of new perovskites La2MRhO6 (M = Mg, Zn)

New perovskite type oxides of Rh(IV) (La2MRhO6; M = Mg, Zn) have been prepared by conventional techniques used in solid state chemistry. Rietveld refinements from X-ray diffraction data confirm the space group P21/n for both compounds. This low symmetry is a result of two effects: (i) slight GdFeO3-type distorsion of the ideal perovskite structure due to the size of six-fold co-ordinated B cations and (ii) 1/1 ordering of the B-cations. The observed lattice parameters are a = 0.557 9(1) nm, b = 0.561 5(1) nm, c = 0.789 8(1) nm, = 90.01(6)° for La2MgRhO6 and a = 0.558 4(3) nm, b = 0.565 9(3) nm, c = 0.791 4(4) nm, = 90.0(2)° for La2ZnRhO6, respectively. The magnetic moment localized on Rh(IV) is strongly reduced compared to the spin-only value. The reasons for this phenomenon are discussed accounting for theoretical work on the magnetic behaviour of 2T2g-terms. However, a weak antiferromagnetic exchange interaction is observed. Both materials are intrinsic semiconductors.