Elastic properties of superionic cubic silver sulfide ß-Ag2S.

Measurement of heat capacity of superionic silver sulfide (argentite ß-Ag2S) revealed that nanocrystalline argentite has a higher heat capacity than coarse-crystalline argentite. It is shown that the heat capacity of nanocrystalline substances includes an additional positive contribution caused by the limitation of the phonon spectrum on the part of low and high frequencies due to a small particle size. The estimation of this contribution on the basis of experimental differences in the heat capacities of nano- and coarse-crystalline argentite ß-Ag2S in the region of its existence, 470-850 K, made it possible to determine for the first time the velocities of propagation of longitudinal and transverse elastic vibrations cl and ct and elastic stiffness constants c11, c12 and c44. It is established that with a rise in temperature the elastic characteristics of argentite decrease. The anisotropy of elastic properties of cubic argentite was analyzed, and the crystal lattice directions corresponding to the largest and the least values of elastic moduli were determined. An increase of the temperature from 470 to 850 K leads to a small decrease of the elastic anisotropy of cubic ß-Ag2S argentite.

Direct TEM observation of the "acanthite α-Ag2S-argentite ß-Ag2S" phase transition in a silver sulfide nanoparticle.

For the first time, the α-Ag2S (acanthite)-ß-Ag2S (argentite) phase transition in a single silver sulfide nanoparticle has been observed in situ using a high-resolution transmission electron microscopy method in real time. Colloid solutions of Ag2S nanoparticles and nanostructured powders of silver sulfide have been synthesized by one-stage chemical bath deposition from an aqueous solution of silver nitrate, sodium sulfide and sodium citrate. Ag2S nanoparticles were heated to different temperatures directly in an electronic microscope by regulating the energy of the electron beam. This allowed observation of the transition of acanthite into argentite and the reversible transition of argentite into acanthite in real time, and this phase transition to be filmed. Temperature dependence of the lattice constant a arg of argentite ß-Ag2S in the temperature range 448-723 K is established by in situ high-temperature X-ray diffraction. The orientation relationships between the monoclinic acanthite α-Ag2S and the body-centered cubic argentite ß-Ag2S are determined. It is shown that the possible distances between silver atoms in cubic argentite, in contrast to those in acanthite, are too small for the positions of the metal sublattice to be occupied by Ag atoms with a probability equal to 1.

Correction: High-temperature X-ray diffraction and thermal expansion of nanocrystalline and coarse-crystalline acanthite α-Ag2S and argentite ß-Ag2S.

Correction for 'High-temperature X-ray diffraction and thermal expansion of nanocrystalline and coarse-crystalline acanthite α-Ag2S and argentite ß-Ag2S' by S. I. Sadovnikov et al., Phys. Chem. Chem. Phys., 2016, 18, 4617-4626.

High-temperature X-ray diffraction and thermal expansion of nanocrystalline and coarse-crystalline acanthite α-Ag2S and argentite ß-Ag2S.

An in situ study of thermal expansion of polymorphic phases of coarse-crystalline and nanocrystalline silver sulfide - monoclinic acanthite α-Ag2S and cubic argentite ß-Ag2S - has been carried out for the first time using the high-temperature X-ray diffraction method. The temperature dependencies of the unit cell parameters of acanthite and argentite in the interval of 300-623 K have been determined, and the thermal expansion coefficients of acanthite and argentite have been found. It is shown that the observed difference in the thermal expansion coefficients for nano- and coarse-crystalline acanthite is due to the small particle size of nanocrystalline silver sulfide leading to the growth of anharmonicity of atomic vibrations. It is established by differential thermal analysis that a reversible polymorphic acanthite-argentite phase transformation takes place at ∼449-450 K and the phase transformation enthalpy is equal to ∼3.7-3.9 kJ mol(-1).

An in situ high-temperature scanning electron microscopy study of acanthite-argentite phase transformation in nanocrystalline silver sulfide powder.

For the first time, the α-Ag2S (acanthite)-ß-Ag2S (argentite) phase transformation in nanocrystalline and coarse-crystalline powders of silver sulfide has been observed in situ by the scanning electron microscopy method in real-time. The argentite crystals are formed on the surface of acanthite particles as a result of electron-beam heating. According to the differential thermal analysis data, the transformation occurs at a temperature of ∼449-450 K, and the enthalpy of transformation is equal to ∼3.7-3.9 kJ mol(-1). The presence of α-Ag2S (acanthite)-ß-Ag2S (argentite) phase transformation is confirmed in situ by high-temperature X-ray diffraction data.

Nonstoichiometry of nanocrystalline monoclinic silver sulfide.

Powders of silver sulfide have been synthesized by chemical bath deposition from aqueous solutions of silver nitrate and sodium sulfide in the presence of sodium citrate or EDTA-H2Na2. Colloid solutions have been prepared by a chemical condensation method from the same aqueous solutions. Synthesized silver sulfide nanopowders have a monoclinic (space group P21/c) acanthite-type structure but the occupancy of the metal sublattice sites by Ag atoms is smaller than 1. Unlike coarse-crystalline silver sulfide Ag2S, silver sulfide nanopowders with particles sizes of less than ∼50 nm are nonstoichiometric, contain vacant sites in the metal sublattice and have a composition of ∼Ag1.93S.