High-pressure synthesis of the polymorph of layer structured compounds MNX(M = Zr, Hf; X = Cl, Br, I)

Transition metal nitride halides MNX (M = Zr, Hf; X = Cl, Br I) have two ty pes of layer structured polymorphs, the alpha-form with the FeOCl type and the beta-form with the SmSI type. Both polymorphs consist of corrugated dou ble M-N layers sandwiched between halogen layers, but with different atomic arrangements within the layers. The beta-form had been considered to be a high-temperature polymorph, because some beta-forms were obtained by therma l treatment of the corresponding alpha-forms. Hen, the alpha-form was succe ssfully transformed into the beta-form under high-pressure and high-tempera ture conditions; the new members of the beta-form were prepared for the fir st time from alpha-HfNBr, alpha-ZrNI, and alpha-HfNI using a high pressure of 3-5 GPa at 900 degrees C. The beta-form should be characterized as the h igh-pressure form rather than the corresponding high-temperature polymorph. This is the first high-pressure study on the polymorphs of metal nitride s ystems.