Disintegration of a coarse-grained vanadium monoxide VO<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> powder

A coarse-grained powder of nonstoichiometric cubic vanadium monoxide VO _y is disintegrated in a Retch PM 200 planetary ball mill. Milling of the coarse-grained vanadium monoxide powder VO _y at a rate of rotation of 500 rpm for 2 h significantly broadens diffraction lines, and the crystal structure of vanadium monoxide VO_1.00 after milling remains the same. High-resolution scanning electron microscopy and X-ray diffraction studies of the microstructure of vanadium monoxide demonstrate that high-energy milling can produce vanadium monoxide powders with an average crystallite size of 23 ± 10 nm. The vanadium monoxide produced by milling has a crystallite size that is half the crystallite size in the titanium monoxide produced by severe plastic deformation.     

Determination of the lattice parameters near the first-order phase transition in the Pb<Subscript>3</Subscript>(PO<Subscript>4</Subscript>)<Subscript>2</Subscript> ferroelastic

A new method for determining the lattice parameters near the first-order phase transition is proposed. The method is based on the model of a thin phase boundary and a linear dependence between the lattice parameters within a phase.     

DO<Subscript>3</Subscript>-type cubic structure of nonstoichiometric vanadium monoxide

An X-ray diffraction study indicates that nonstoichiometric vanadium monoxide VO _y ≡ V _x O _z (y = z/x) has a cubic structure of the DO₃ type (space group Fm $ \bar 3 $ \bar 3 m), where vanadium atoms are not only at the 4(a) sites of the metal fcc sublattice, but also at the tetrahedral 8(c) sites. This circumstance fundamentally distinguishes monoxide VO _y from strongly nonstoichiometric MX _y compounds with the B1 structure and the same space group, where atoms M and X and structural vacancies ▪ and ▭ of the metal and nonmetal sublattices, respectively, are distributed over the 4(a) and 4(b) sites. The dependence of the filling factor q of the tetrahedral interstices by vanadium atoms on the relative content y of oxygen in VO _y has been obtained. It has been shown that the composition of cubic vanadium monoxide should be represented as VO _y ≡ V _x O _z ≡ V _{x − 2q} V_2q ^(t)▪_{1 − x + 2q} O _z ▭_{1 − z} , taking into account the structure.     

Atomic displacements in the V<Subscript>52</Subscript>O<Subscript>64</Subscript> superstructure and the short-range order in superstoichiometric cubic VO<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> vanadium monoxide with metal vacancies

Atomic displacements in the lattice of the tetragonal V₅₂O₆₄ superstructure have been experimentally determined. It has been found that atomic displacement waves, which are attributed to the formation of the short-range displacement order, appear in the vanadium and oxygen sublattices of this superstructure. It has been shown that the V₅₂O₆₄ superstructure is formed on the basis of disordered superstoichiometric cubic vanadium monoxide with the short-range order in the metallic sublattice. The character of the short-range order is such that vanadium atoms occupying tetrahedral positions are in the environment of four vacant sites of the vanadium sublattice. This means that the superstoichiometric VO_>1.0 vanadium monoxide has a cubic structure differing from the B1-type structure characteristic of most of the strongly nonstoichiometric cubic compounds MX _y (X = C, N, O) of transition metals.     

Features of acoustic wave propagation near the structural phase transition in the La<Subscript>0.875</Subscript>Sr<Subscript>0.125</Subscript>MnO<Subscript>3</Subscript> manganite

Features of the phase transition from the disordered state to the ordered orbital state in a La_0.875Sr_0.125MnO₃ single crystal, caused by the cooperative Jahn-Teller effect, have been investigated. A significant change in the acoustic wave parameters in the entire range of cooperative distortion of the structure is revealed. Application of an external magnetic field shifts the structural phase transition to low temperatures.     

V<Subscript>52</Subscript>O<Subscript>64</Subscript> tetragonal superstructure of cubic vanadium monoxide with vacancies in the metal sublattice

The atom-vacancy ordering of cubic vanadium monoxide VO_1.29, which has basis cubic structure B1 and structural vacancies in the metal sublattice, has been studied using the x-ray diffraction method. It has been shown that the formation of the tetragonal (space group I4₁/amd) ordered phase V₅₂O₆₄ of cubic vanadium monoxide VO_y proceeds as a first-order phase transition through the disorder-order channel including 22 nonequivalent superstructure vectors of four stars {k ₁₀}, {k ₄}, {k ₃}, and {k ₂}. The distribution function of the vanadium atoms in the V₅₂O₆₄ tetragonal superstructure has been calculated.     

Performance comparisons of passively Q-switched intracavity-frequency-doubled Nd:Gd<Subscript>0.19</Subscript>Y<Subscript>0.81</Subscript>VO<Subscript>4</Subscript> and Nd:Gd<Subscript>0.83</Subscript>Y<Subscript>0.17</Subscript>VO<Subscript>4</Subscript> lasers at 671 nm

Performance comparisons of laser-diode pumped passively Q-switched intracavity-frequency-doubled Nd:Gd_0.19Y_0.81VO₄ and Nd:Gd_0.83Y_0.17VO₄ lasers at 671 nm are demonstrated for the first time to our knowledge. KTP crystal is used as the frequency doubling material and V:YAG crystal as the saturable absorber with initial transmission of 89%. The dependences of average output power, pulse width, pulse repetition rate, single-pulse energy and peak power on incident pump power are measured and contrasted. The experimental results show that, Nd:Gd_0.83Y_0.17VO₄ laser has more excellent properties than Nd:Gd_0.19Y_0.81VO₄ laser at 671 nm.     

Ferroelectric phase transition of Nb-doped Bi<Subscript>3.25</Subscript>La<Subscript>0.75</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> ceramics

Bi_3.25La_0.75Ti_3-yNb_yO₁₂ (y=0.0, 0.03, 0.09, 0.15, 0.21) were synthesized using the solid-state reaction method. The effects of Nb doping on ferroelectric properties were studied through dielectric and P-E measurements. The value of P_r increases with increasing Nb content. Bi_3.25La_0.75Ti_3-yNb_yO₁₂ ceramics exhibit a maximum remanent polarization of P_r=27 μC/cm² at an Nb content of y=0.09. These results indicate that Nb doping can improve the ferroelectric properties of BLT ceramics. The Curie temperature, T_c, decreased with increasing Nb-content, and the ferroelectric phase transition of BLTNy is a second-order transition without thermal hysteresis. PACS 77.55.+f; 77.80.-e; 77.22.Jp     

Superproton phase transition in a K<Subscript>3</Subscript>H(SO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystal

The conductivity and heat capacity of tripotassium hydrogen trisulfate single crystals were studied. It was shown that these single crystals undergo a solid-solid phase transformation upon heating with the formation of new phases, and that their properties change considerably. Anomalies related to the sorption and desorption of water vapor were observed in the formed multiphase system. Such anomalies were observed for the first time in measurements of the true heat capacity.     

Phase behaviour and superionic phase transition in K<Subscript>3</Subscript>H(SeO<Subscript>4</Subscript>)<Subscript>2</Subscript>

The phase behaviour of K₃H(SeO₄)₂ (TKHSe) above room temperature has been studied by differential scanning calorimetric (DSC), thermogravimetric analysis (TGA), simultaneous thermogravimetric and mass spectroscopy analysis (TG-MS), impedance spectroscopy (IS) and X-ray powder diffraction (XRD). Our results show that the previously claimed superionic phase transition in TKHSe at around 388 K (114.85 °C) is also the onset temperature of a slow thermal dehydration that occurs at reaction sites distributed over the surface of the crystal. That is, we propose that the TKHSe undergoes simultaneously a superionic phase transition and a decomposition process with a very slow reaction rate that is evident when the sample is pulverized to fine powder, both starting at the same temperature. As a matter of fact, we observe a decrease of the magnitude of the dc-conductivity on successive thermal runs in powdered sample attributed to sample decomposition that starts at the surface of the TKHSe grains, but the jump in conductivity is only a consequence of the order–disorder transition in the TKHSe phase that remains inside the grains.     

Pressure-induced electron spin transition in the paramagnetic phase of the GdFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> Heisenberg magnet

The HS → LS spin crossover effect (high-spin → low-spin transition) induced by high pressure in the range 45–53 GPa is observed in trivalent Fe³⁺ ions in the paramagnetic phase of a Gd⁵⁷Fe₃(BO₃)₄ gadolinium iron borate crystal. This effect is studied in high-pressure diamond-anvil cells by two experimental methods using synchrotron radiation: nuclear resonant forward scattering (NFS) and Fe K _β high-resolution x-ray emission spectroscopy (XES). The manifestation of the crossover in the paramagnetic phase, which has no order parameter to distinguish between the HS and LS states, correlates with the optical-gap jump and with the insulator-semiconductor transition in the crystal. Based on a theoretical many-electron model, an explanation of this effect at high pressures is proposed.     

Pressure-induced phase transition in the cubic ScF<Subscript>3</Subscript> crystal

Pressure-induced phase transitions in the ScF₃ crystal were studied using synchrotron radiation diffraction, polarization microscopy, and Raman spectroscopy. The phase existing in the range 0.6–3.0 GPa is optically anisotropic; its structure is described by space group R ₃ c (Z = 2), and the transition is due to rotation of ScF₆ octahedra around a threefold axis. The pressure dependence of the structural parameters and angle of rotation are determined. The number of Raman spectral lines corresponds to that expected for this structure; above the phase transition point, a recovery of soft modes takes place. At a pressure of 3.0 GPa, a transition occurs to a new phase, which remains metastable as the pressure decreases. The results are interpreted using an ab initio method based on the Gordon-Kim approach.     

Synthesis and structural,magnetic, and resonance properties of the LiCuFe<Subscript>2</Subscript>(VO<Subscript>4</Subscript>)<Subscript>3</Subscript> compound

Complex studies have been performed for the structural, static magnetic, and resonance properties of a new magnet LiCuFe₂(VO₄)₃ prepared by solid-phase synthesis. The temperature dependence of the susceptibility has an anomaly at temperature T_max = 9.6 K. At high temperatures, the LiCuFe₂(VO₄)₃ sample is in the paramagnetic state described by the Curie–Weiss law at T > 50 K and mainly determined by iron ions with effective magnetic moment μ_eff(exp) = 8.6μ_B per formula unit. At low temperatures, a long-range magnetic order is observed in the magnetic subsystem of the sample; the order is predominantly characterized by the antiferromagnetic exchange interaction and high frustration level. The exchange interaction parameters are estimated in a six-sublattice representation of the LiCuFe₂(VO₄)₃ magnet. It is shown that the LiCuFe₂(VO₄)₃ compound is an antiferromagnet with strong intrachain and frustrating interchain exchange interactions.     

Anomalous slowing down of the longitudinal spin relaxation near the antiferromagnetic phase transition in LaMnO<Subscript>3</Subscript> manganite

It has been found that, instead of the expected critical acceleration of the longitudinal spin relaxation near the Néel temperature in stoichiometric LaMnO₃ samples, the relaxation is sharply retarded. This slowing down is similar to that observed earlier in doped manganites with ferromagnetic ordering.     

Ferroelastic phase transition in crystalline K<Subscript>3</Subscript>Na(CrO<Subscript>4</Subscript>)<Subscript>2</Subscript>: Acoustic studies

Ultrasonic studies of the temperature behavior of the velocity and damping of sound for the xx and zz longitudinal and yx and zx transverse waves in K₃Na(CrO₄)₂ have been carried out in the temperature interval 185–295 K, which includes the region of the ferroelastic phase transition. The acoustic parameters for both shear and longitudinal waves were found to have anomalies in the region of the phase transition with a Curie temperature of 235.5 K. A theoretical analysis of the softening of the elastic moduli c₄₄ and c₆₆ was performed on the basis of the Landau expansion in terms of the strain tensor components ?₄ and (?₂-?₁)/2 considered as the linearly coupled primary and secondary order parameter, respectively. The absolute values of the elastic moduli c₁₁, c₃₃, c₄₄, c₆₆, c₁₂, and c₁₄ at 295 K were calculated.     

All-solid-state Nd:Gd<Subscript>0.18</Subscript>Y<Subscript>0.82</Subscript>VO<Subscript>4</Subscript>-LBO green laser at 532 nm

We report a continuous-wave (CW) coherent green radiation at 532 nm by intracavity frequency doubling generation of 1064 nm Nd:Gd_0.18Y_0.82VO₄ laser. With incident pump power of 18.2 W, output power of 1.08 W at 532 nm has been obtained using a 5 mm-long KTP crystal. The optical conversion efficiency was up to 5.9%. At the output power level of 1.08 W, the output stability is better than 5%. The beam quality M² values were equal to 1.26 and 1.12 in X and Y directions, respectively.     

Critical behavior of La<Subscript>0.825</Subscript>Sr<Subscript>0.175</Subscript>MnO<Subscript>2.912</Subscript> anion-deficient manganite in the magnetic phase transition region

For La _0.825 ³⁺ Sr _0.175 ² +Mn³⁺O _2.912 ^2? anion-deficient manganite, the specific magnetization, the dynamic magnetic susceptibility, and the heat capacity are investigated. This material is found to be an inhomogeneous ferromagnet below the Curie point T _C ≈ 122 K, which is much lower than the Curie point determined for the stoichiometric composition (T _C ≈ 268 K). An increase in magnetic field by two orders of magnitude leads to an increase in the Curie temperature by ΔT ≈ 12 K. The presence of oxygen vacancies leads to the frustration of a part, namely, V _fr ≈ 22%, of the indirect Mn³⁺-O-Mn³⁺ exchange interactions, but the spin glass state is not realized. The ferromagnetic matrix of the material under study is characterized by a scatter in the exchange interaction intensities. The heat capacity is found to exhibit an anomalous behavior. Based on the Banerjee magnetic criterion, it is established that the ferromagnet-paramagnet transition observed for La _0.825 ³⁺ Sr _0.175 ²⁺ Mn³⁺O _2.912 ^2? anion-deficient manganite is a second-order thermodynamic phase transition. The mechanism and origin of the critical behavior of the system under investigation are discussed.     

Characteristics of diode-pumped dual-loss-modulated Q-switched and mode-locked Nd:Lu<Subscript>0.15</Subscript>Y<Subscript>0.85</Subscript>VO<Subscript>4</Subscript> laser

A diode-pumped dual-loss-modulated Q-switched and mode-locked (QML) Nd:Lu_0.15Y_0.85VO₄ laser with acousto-optic (AO) modulator and Cr⁴⁺:YAG saturable absorber is presented. The stable QML laser pulse with high peak power and complete modulation depth has been obtained. The QML laser characteristics such as the pulse width, single-pulse energy etc. have been measured for different small-signal transmissions (T ₀) of Cr⁴⁺:YAG, different reflectivity (R) of output coupler and modulation frequencies of the AO modulator (f _p ). The results show that the pulse energy increases with decreasing f _p and increasing T ₀, while the pulse width decreases with decreasing f _p and increasing T ₀. At f _p = 10 kHz, R = 90%, and T ₀ = 91%, the highest pulse energy and peak power of mode-locked pulses is obtained.     

Ordered monoclinic vanadium suboxide V<Subscript>14</Subscript>O<Subscript>6</Subscript>

The monoclinic (space group C2/m) superstructure of the suboxide V₁₄O₆, which is formed as a result of the atomic and vacancy ordering of the tetragonal solid solution of oxygen in vanadium, is investigated using X-ray diffraction and symmetry analysis. The monoclinic suboxide V₁₄O₆ is observed in the vanadium oxide samples VO_0.57, VO_0.81, and VO_0.86 synthesized at 1770 K and the samples VO _y (0.87 ≤ y ≤ 0.98) additionally annealed at 1470 K after the synthesis. It is established that the channel of the disorder-order phase transition associated with the formation of the monoclinic suboxide V₁₄O₆ includes six superstructure vectors belonging to three non-Lifshitz stars of one type {k ₁}. The distribution function of the oxygen atoms in the monoclinic superstructure of the suboxide V₁₄O₆ is calculated. It is demonstrated that the displacements of vanadium atoms distort the body-centered tetragonal metal sublattice, thus preparing the formation of the facecentered cubic sublattice and the transition from the suboxide V₁₄O₆ to the cubic vanadium monoxide with the B1 structure.     

Growth and spectral properties of self-frequency doubling crystal,Nd:Ca<Subscript>9.03</Subscript>Na<Subscript>1.08</Subscript>La<Subscript>0.62</Subscript>(VO<Subscript>4</Subscript>)<Subscript>7</Subscript>

The non-doped and doped Nd³⁺ of Ca_9.03Na_1.08La_0.62(VO₄)₇ crystals were grown by the Czochralski technique. The effective segregation coefficients of Na⁺ and Nd³⁺ ions in the crystal were measured to be about 0.5 and 1.1, respectively. The XPS analysis of Ca_9.03Na_1.08La_0.62(VO₄)₇ crystal indicates that the vanadium in the crystal is a mixture of V⁴⁺ (1.46 at. %) and V⁵⁺ (98.54 at. %). The hardness of Nd:Ca_9.03Na_1.08La_0.62(VO₄)₇ crystal is about 383.1 VDH. Nd:Ca_9.03Na_1.08La_0.62(VO₄)₇ crystal exhibits similar thermal expansion coefficients along the a (11.2×10^-6 K^-1) and c (13.7×10^-6 K^-1) axes, indicating a low thermal expansion anisotropy (α_c/α_a≈1.2). The qualitative frequency-doubling experiment shows that the doping of Na⁺ ion can help reduce the scattering of frequency-doubling light, and the intensity of SHG for Ca_9.03Na_1.08La_0.62(VO₄)₇ crystal is found to be about 3.5 times as large as that of KDP. The polarized absorption and fluorescence spectra are analyzed based on Judd–Ofelt theory, which exhibits that the π-polarized absorption and stimulated emission cross sections are 6.07×10^-20 cm² with an FWHM 12.0 nm at 810 nm and 1.42×10^-19 cm² at 1069 nm, respectively. The fluorescence lifetime is 115 μs at room temperature. All the results indicate that Nd:Ca_9.03Na_1.08La_0.62(VO₄)₇ crystal is a candidate of self-frequency doubling laser material. PACS 42.62.Fi; 42.70.Mp; 81.10.Fq