DOMAIN SEPARATION IN SINGLE-CRYSTAL (ND4)2CU(SO4)2.6D2O STUDIED BY X-RAY-DIFFRACTION AT HIGH-RESOLUTION

Analysis of asymmetric line shapes of the synchrotron-generated x rays diffracted at 12 K from ca. 85% deuterated single-crystal copper ammo nium Tutton salt, (ND4)2Cu(OD2)6(SO4)2, shows a breakdown in translati onal order that we interpret as regions of two monoclinic phases segre gated on a scale of 100-200 unit cells in fixed relative orientation. The separation and orientation can be understood as the best epitactic fit between crystals differing very slightly in Jahn-Teller distortio n at the Cu site. Previous studies have shown two distinct much larger Jahn-Teller distortions at the Cu site to be close in free energy. At 180 K more perfect order is regained, and the process is reversible. This indicates that the phase separation is a property of the macrosco pically uniform single crystal, and is not caused by the crystal exist ing in two large as-grown parts differing in deuteration. At higher te mperatures, to 320 K, linewidths again increase, but then with no asym metry. The decrease in domain size may be connected with an approachin g order-disorder transition, which is also reflected in changes in the cell constants and in the ammonium-ion librational motions.