Uniaxial Orientational Order–Disorder Transitions in Diammine Magnesium Halides, Mg(ND3)2Cl2 and Mg (ND3)2Br2, Investigated by Neutron Diffraction

✍ Scribed by A. Leineweber; H. Jacobs; P. Fischer; G. Böttger

• Publisher: Elsevier Science
• Year: 2001
• Tongue: English
• Weight: 461 KB
• Volume: 156
• Category: Article
• ISSN: 0022-4596
• DOI: 10.1006/jssc.2000.9028

✦ Synopsis

Neutron powder di4raction on Mg(ND 3 ) 2 Cl 2 and Mg(ND 3 ) 2 Br 2 revealed as a function of temperature uniaxial orientational order+disorder behavior of the ND 3 groups. The crystal structures of both compounds are built up from chains of octahedra 1 [Mg(NH 3 ) 2 X 4/2 ] with X ‫؍‬ Cl and Br arranged in di4erent ways relative to each other. At ambient temperatures (X ‫؍‬ Cl) and 270 K (X ‫؍‬ Br) the ND 3 groups are disordered with respect to a rotation about the bond Mg+N. The D atom density is well described by a fourfold split position, each D 99site:: connecting an N with an X atom: Mg(ND 3 ) 2 Cl 2 , Cmmm, a ‫؍‬ 8.1828(6) A s , b ‫؍‬ 8.2007(6) A s , c ‫؍‬ 3.7543(2) A s , R(F 2 ) Bragg ‫؍‬ 5.9%; Mg(ND 3 ) 2 Br 2 , Pbam, a ‫؍‬ 5.9714(2) A s , b ‫؍‬ 11.9175(3) A s , c ‫؍‬ 3.98477(8) A s , R(F 2 ) Bragg ‫؍‬ 7.9%. In both cases the c axis corresponds to the direction of the chains 1 [Mg(NH 3 ) 2 X 4/2 ]. At low temperatures (8 K (X ‫؍‬ Cl) and 1.5 K (X ‫؍‬ Br)) both compounds are ordered with respect to the ND 3 groups: They are arranged antiferroelectrically on either side of the chains

The symmetry is lowered compared to the situation at ambient temperatures and 270 K respectively, which involves in both cases a doubling of the orthorhombic c axis: Mg(ND 3 ) 2 Cl 2 , Ibmm, a ‫؍‬ 8.1319(3) A s , b ‫؍‬ 8.1338(3) A s , c ‫؍‬ 7.4410(2) A s , R(F 2 ) Bragg ‫؍‬ 5.9%; Mg(ND 3 ) 2 Br 2 , Pnam, a ‫؍‬ 5.92837(8) A s , b ‫؍‬ 11.8448(2) A s , c ‫؍‬ 7.9117(1) A s , R(F 2 ) Bragg ‫؍‬ 5.0%. Detailed evaluation of neutron di4raction data of Mg(ND 3 ) 2 Cl 2 as a function of temperature (50 K< T < 150 K) characterizes the phase transition as continuous with T t +135 K. 2001 Academic Press