Nanocrystallinity as a Route to Metastable Phases: Rock Salt ZnO

Abstract

A synthesis route to rock-salt zinc oxide (rs-ZnO), high-pressure phase metastable at ambient conditions, has been developed. High-purity bulk nanocrystalline rs-ZnO has been synthesized from wurtzite (w) ZnO nanopowders at 7.7 GPa and 770-820 K and for the first time recovered at normal conditions. Structure, phase composition and thermal phase stability of recovered rs-ZnO have been studied by synchrotron X-ray powder diffraction and X-ray absorption spectroscopy (XANES and EXAFS) at ambient pressure. Phase purity of rs-ZnO was achieved by usage of w-ZnO nanoparticles with narrow size distribution as a pristine material synthesized by various chemical methods. At ambient pressure rs-ZnO could be stable up to 360 K. The optical properties of rs-ZnO have been studied by conventional cathodoluminescence in high vacuum at room and liquid-nitrogen temperatures. The nanocrystalline rs-ZnO at 300 and 77 K has shown bright blue luminescence at 2.42 and 2.56 eV, respectively. KEYWORDS cubic zinc oxide, nanocrystals, high-pressure synthesis, phase transition, luminescent properties.

On the thermodynamic aspect of zinc oxide polymorphism: calorimetric study of metastable rock salt ZnO

“…The details of high-pressure synthesis and characterization of recovered samples have been described elsewhere [10]. Nanocrystalline bulk w-ZnO to be used as a reference sample was produced by a reverse phase transition of nanocrystalline bulk rs-ZnO as a result of heating from 300 to 523 K with heating rate of 1ºK/min with DSC monitoring of the transition.…”

“…Since the discovery of high-pressure polymorph of zinc oxide, rock salt ZnO in 1962 [1], its synthesis and properties have been intensively studied [2][3][4][5][6][7][8][9][10][11][12][13][14]. The most of these studies were conducted in situ under high pressure due to the problems with rs-ZnO recovery at ambient conditions.…”

“…Recently, it was found that single-phase rs-ZnO synthesized at 7.7 GPa and 800 K can be completely recovered at normal conditions in the form of macroscopic (> 100 mm 3 ) bulks [10]. At ambient pressure these bulks are kinetically stable up to 370 K which allows the direct measuring of thermochemical properties of metastable rs-ZnO by conventional calorimetry.…”

Order and disorder around Cr3+ in chromium doped persistent luminescent AB2O4 spinels

“…[9][10][11][12][13][14] Tunable, coherent, polarized radiation along with a high photon flux obtained from a synchrotron makes it favourable over conventional spectroscopic methods. 15,16 Owing to the applicability of XAFS to crystalline as well as amorphous materials, the technique can be employed to characterize a wide variety of materials such as optical materials, 9,11,17 magnetic materials, 13 ancient and historical materials, 15 et cetera.…”

High-coercivity magnetism in nanostructures with strong easy-plane anisotropy

“…[3][4][5]7 Furthermore, the nonequilibrium fabrication process used for nanoclusters or particles can stabilize metastable magnetic materials and often leads to new crystal structures including those with very high magnetocrystalline anisotropies. 8,9 In this communication, we have used a non-equilibrium cluster deposition method to synthesize a rare-earth-free magnetic material Co 3 Si in the form of nanoparticles and verified its crystal structure, which is a high-temperature metastable phase according to the bulk phase diagram and has not been produced previously. While density-functional theory (DFT) calculations are used to show that Co 3 Si crystal has an easy-plane anisotropy, having a high K 1 % À64 Merg/cm 3 , we prove using experiments and micromagnetic analysis that this negative anisotropy translates into high coercivities (17.4 kOe at 10 K and 4.3 kOe at 300 K) upon nanostructuring.…”