Uniaxial orientational order-disorder transitions in diammine magnesium halides, Mg(ND3)(2)Cl-2 and Mg(ND3)(2)Br-2, investigated by neutron diffraction

Neutron powder diffraction on Mg(ND3)(2)Cl-2 and Mg(ND3)(2)Br-2 revealed as a function of temperature uniaxial orientational order-disorder behavior o f the ND3 groups. The crystal structures of both compounds are built up fro m chains of octahedra (1)(infinity)[Mg(NH3)(2)X-4/2] with X = Cl and Br arr anged in different ways relative to each other. At ambient temperatures (X = Cl) and 270 K (X = Br) the ND3 groups are disordered with respect to a re lation about the bond Mg-N. The D atom density is well described by a fourf old split position, each D "site" connecting an N with an X atom: Mg(ND3)(2 )Cl-2, Cmmm, a = 8.1828(6) Angstrom, b = 8.2007(6) Angstrom, c = 3.7543(2) Angstrom, R(F-2)(Bragg) = 5.9%; Mg(ND3)(2)Br-2, Pbam, a = 5.9714(2) Angstro m, b = 11.9175(3) Angstrom, c = 3.98477(8) Angstrom, R(F-2)(Bragg) = 7.9%. In both cases the c axis corresponds to the direction of the chains (1)(inf inity)[Mg(NH3)(2)X-4/2]. At low temperatures (8 K (X = Cl) and 1.5 K (X = B r)) both compounds are ordered with respect to the ND, groups: They are arr anged antiferroelectrically on either side of the chains (1)(infinity)[Mg(N H3)(2)X-4/2]. The symmetry is lowered compared to the situation at ambient temperatures and 270 K respectively, which involves in both cases a doublin g of the orthorhombic c axis: Mg(ND3)(2)Cl-2, Ibmm, a = 8.1319(3) Angstrom, b = 8.1338(3) Angstrom, c = 7.4410(2) Angstrom, R(F-2)(Bragg) = 5.9%; Mg(N D3)(2)Br-2, Pnam, a = 5.92837(8) Angstrom, b = 11.8448(2) Angstrom, c = 7.9 117(1) Angstrom, R(F-2)(Bragg) = 5.0%. Detailed evaluation of neutron diffr action data of Mg(ND3)(2)Cl-2 as a function of temperature (50 K< T< 150 K) characterizes the phase transition as continuous with T-t approximate to 1 35K. (C) 2001 Academic Press.