A novel material is obtained by modifying pristine YBCO superconductor via simultaneous partial substitution of Sr for Ba and Se for O. The intended stoichiometry was YBa(2-x)YBa(2-x)Sr(x)Cu(3)O(7-x)O7-xSe(x) with x = 0.5, 0.75 and 1; and x approximate to 1 yielded the most interesting results considered in detail. We confirmed that Se atoms indeed enter the superconductor lattice cell. The obtained polycrystalline material contains five phases, with the major phase (>44.%) being YBa1.4YSr0.6Cu3O6Se0.5. The synthesized superconductor demonstrates unique properties, including (i) two superconducting transitions with T-c1 approximate to 36K and T-c2 approximate to 13K yielding the Wohlleben effect; (ii) strong-coupling d-wave superconductivity (with 2 Delta/(k(B)T(c)) approximate to 5); (iii) Schottky anomaly in the heat capacity; and, most importantly, (iv) a very unusual anomaly in the heat capacity which can be associated with the quantum criticality typically observable at much higher fields. The fact that the quantum criticality is visible in weaker fields (similar to 7T) may open opportunities for exploration of the interplay between superconductivity and pair density waves by wide research community particularly when its pure phase is available. To facilitate this, the stability of single-phase YBa1.4Sr0.6Cu3O6Se0.5 is analyzed using machine learning methods.

Millimeter-sized crystals of sodium hexafluorosilicate Na2SiF6 have been grown by the hydrothermal method. X-ray diffraction analysis revealed that Na2SiF6 samples are twinned according to the merohedral law and crystallize in the sp. gr. Р321 with the following unit-cell parameters at 295 K: 〈a〉 = 8.8582(12) Å, 〈c〉 = 5.0396(11) Å, 〈V〉 = 342.47(17) Å3 (averaged results of repeated measurements). A multitemperature diffraction study of Na2SiF6 was performed; the results obtained were used to calculate the temperature dynamics of the optical properties of crystals. Structural similarity was revealed between Na2SiF6 crystals and crystals of the langasite family La3Ga5SiO14. This made it possible to explain the optical activity of Na2SiF6 by considering the electron density helices similar to those in langasite, twisted around the threefold symmetry axis passing through the origin of the Na2SiF6 cell. The kinks in the temperature dependences of the refractive index and rotation of the plane of polarization of light are explained taking into account the anomalous features of interatomic interactions along the threefold axis of the unit cell, passing through the Si2(2d) site with the coordinates (1/3, 2/3, z). It was found that main factor affecting the temperature dynamics of optical parameters is the Si2(2d)–F2(6g) distance, which increases abnormally upon cooling.

We report on a novel material obtained by modifying pristine YBCO superconductor in solid phase synthesis via simultaneous partial substitution of Ba by Sr and O by Se. Simultaneous application of EDX and EBSD confirmed that Se atoms indeed enter the crystalline lattice cell. The detailed XRD analysis further confirmed this conclusion and revealed that the obtained polycrystalline material contains 5 phases, with the major phase ($>$30\%) being a cuprate YBa$_{1.4}$Sr$_{0.6}$Cu$_{3}$O$_{6}$Se$% _{0.51}$. The obtained superconductor demonstrates unique properties, including i) two superconducting transitions with $T_{c1}\approx$ 35 K (granular surface phase) and $T_{c2}\approx$ 13 K (bulk granular phase) - this granular phase arrangement naturally yields the Wohlleben effect; ii) reentrant diamagnetism and resistive state; iii) strong paramagnetism with Curie-Weiss behavior (% $\theta_{CW} \approx$ 4 K) and the ferromagnetic phase overruled by superconductivity; iv) Schottky anomaly visible in the heat capacity data and most likely delivered by small clusters of magnetic moments. Thorough analysis of the heat capacity data reveals a strong-coupling $d-$wave pairing in its bulk phase (with $2\Delta /T_{c}\approx 5$), and, most importantly, a very unusual anomaly in this cuprate. There are reasons to associate this anomaly with the quantum criticality observed in traditional cuprate superconductors at much higher fields (achievable only in certain laboratories). In our case, the fields leading to quantum criticality are much weaker ($\sim $7-9 T) thus opening avenues for exploration of the interplay between superconductivity and pair density waves by the wider research community.

The structure and optical properties of crystals from the langasite family (La1 - xNdх)3Ga5SiO14 with different Nd content were investigated. The rotation of the light polarization plane, ρ, was calculated for these crystals from measured transmission spectra in polarized light. It is shown that for small values of ρ (~3-5 degrees/mm), it is necessary to use transmission spectra not with parallel and crossed polarizers, as is usually done, but at different angles between them, for example ±45°, to obtain better results. Circular dichroism measurements of these crystals were performed. Using Kramers-Kronig relations, the connection between the circular dichroism bands and the rotation of the light polarization plane in the absorption band region was determined. Dispersion curves of ρ values were calculated, taking into account absorption in the wavelength range of 400–1000 nm for crystals (La0.6Nd0.4)3Ga5SiO14, (La0.4Nd0.6)3Ga5SiO14, Nd3Ga5SiO14, and compared with the dispersion of ρ for langasite crystal La3Ga5SiO14. Average refractive indices and optical activity parameters of these crystals were calculated from structural data. It is shown that the dependence of the average refractive indices and ρ values on the parameters of the elementary cell, calculated under the assumption of no absorption, is linear. However, such a linear dependence is not observed for experimental ρ values, which is associated with the influence of absorption and the peculiarities of the structure (nonlinear change in the geometry of optically active regions of electron density upon replacing part of La with Nd).

The optical properties of Sr3NbFe3Si2O14, Ba3NbFe3Si2O14, and Ba3TaFe3Si2O14 single crystals (promising multiferroics belonging to the langasite family (sp. gr. P321, Z = 1)) have been investigated. The crystals have been grown by floating zone melting. The measured and calculated refractive indices of these crystals are compared. The optical activity parameters are calculated based on the structural data. The atomic structures are compared proceeding from the precise X-ray diffraction analysis data. A correlation is established between the structural features and optical properties of the crystals.

The structure and optical properties of langasite family crystals (La 1– x Nd x ) 3 Ga 5 SiO 14 with different Nd contents have been studied. The optical rotation ρ for these crystals has been calculated from the measured polarized-light transmission spectra. It is shown that, at a small ρ value (~3–5 deg/mm), the transmission spectra recorded at different angles between polarizers (e.g., ±45°) should be used to obtain the best results, rather than the transmission spectra at parallel and crossed polarizers, as is generally accepted. The circular dichroism of these crystals has been measured. The relationship between the circular-dichroism bands and the change in the optical rotation near the absorption bands has been determined using the Kramers–Kronig relations. Variances of the parameter ρ with allowance for absorption in the wavelength range of 400–1000 nm have been calculated for (La 0.6 Nd 0.4 ) 3 Ga 5 SiO 14 , (La 0.4 Nd 0.6 ) 3 Ga 5 SiO 14 , and Nd 3 Ga 5 SiO 14 crystals and compared with the variance of ρ for a La 3 Ga 5 SiO 14 langasite crystal. The mean refractive indices and parameters of optical activity of these crystals have been calculated from the structural data. It is shown that the dependences of the mean refractive index and parameter ρ, calculated within the no-absorption approximation, on the unit-cell parameters are linear. Note that the dependence observed for experimental ρ values is not linear, which is related to the influence of absorption and structural features (nonlinear change in the geometry of optically active regions of the electron density when La is partially replaced with Nd).

The crystal structure of samarium iron borate was analyzed with regard to growth conditions and temperature. The inclusion of about 7% Bi atoms in the crystals grown using the Bi2Mo3O12-based flux was discovered and there were no impurities in the crystals grown using the Li2WO4-based flux. No pronounced structural features associated with Bi inclusion were observed. The different absolute configurations of the samples grown using both fluxes were demonstrated. Below 80 K, a negative thermal expansion of the c unit-cell parameter was found. The structure of (Sm0.93Bi0.07)Fe3(BO3)4 belongs to the trigonal space group R32 in the temperature range 90-400 K. A decrease in the (Sm,Bi)-O, Sm-B, Sm-Fe, Fe-O, Fe-B and Fe-Fe distances is observed with a lowering of the temperature, B1-O does not change, B2-O increases slightly and the B2O3 triangles deviate from the ab plane. The strongest decrease in the equivalent isotropic atomic displacement parameters (Ueq) with decreasing temperature is observed for atoms Sm and O2, and the weakest is observed for B1. The O2 atoms have the highest Ueq values, the most elongated atomic displacement ellipsoids of all the atoms and the smallest number of allowed vibrational modes of all the O atoms. The largest number of allowed vibrational modes and the strongest interactions with neighbouring atoms is seen for the B atoms, and the opposite is seen for the Sm atoms. The quadrupole splitting Δ(T) of the paramagnetic Mössbauer spectra increases linearly with cooling. The Néel temperature [TN = 31.93 (5) K] was determined from the temperature dependence of the hyperfine magnetic field Bhf(T), which has a non-Brillouin character. The easy-plane long-range magnetic ordering below TN was confirmed.

Recently, the low-temperature phase of water molecules confined within nanocages formed by the crystalline lattice of water-containing cordierite crystals has been reported to comprise domains with ferroelectrically ordered dipoles within the a, b-planes which are antiferroelectrically alternating along the c-axis. In the present work, comprehensive broad-band dielectric spectroscopy is combined with specific heat studies and molecular dynamics and Monte Carlo simulations in order to investigate in more detail the collective modes and single-particle excitations of nanoconfined water molecules. From DFT-MD simulations we reconstruct the potential-energy landscape experienced by the H2O molecules. A rich set of anisotropic temperature-dependent excitations is observed in the terahertz frequency range. Their origin is associated with the complex rotational/translational vibrations of confined H2O molecules. A strongly temperature dependent relaxational excitation, observed at radio-microwave frequencies for the electric field parallel to the crystallographic a-axis, E||a is analyzed in detail. The temperature dependences of loss-peak frequency and dielectric strength of the excitation together with specific heat data confirm a ferroelectric order-disorder phase transition at T0 ≈ 3 K in the network of H2O dipoles. Additional dielectric data are also provided for polarization E||b, too. Overall, these combined experimental investigations enable detailed conclusions concerning the dynamics of the confined water molecules that develop within their microscopic energy landscapes.

Some optical and mechanical properties of a number of langasite family crystals have been compared. It is shown that electron-density regions in the form of helices, twisted in the opposite directions for the right- and left-handed crystals, make a structural basis of the optical activity. The characteristic surfaces of Young’s modulus have been calculated for some langasite family crystals, and the difference between them is explained from the structural point of view. The fracture anisotropy under Berkovich indentation of the (0001) basal plane has been considered for five crystals. It is shown that crystallographic cracks (straight-line ones and those formed exactly at the indenter angles) are obtained at indenter rotations with a step of 60° from the position in which the indenter angles coincide with the 〈 $$\bar {1}$$ 010〉 directions. The structural basis of piezoelectricity of langasites is established, and a difference in the piezoelectric properties of the right- and left-handed crystals (which is important for manufacture of piezoelectric devices) are shown.

The results of studies of the NdFe3(BO3)(4) by Fe-57 Mossbauer spectroscopy in comparison with the data of single crystal X-ray diffraction measurements are presented. Scanning of the crystal cell parameters in a wide temperature range T = 15-500 K revealed a negative thermal expansion along the c axis and structural anomalies. The temperature dependences of the Mossbauer parameters of hyperfine interaction in the paramagnetic state of NdFe3(BO3)(4) correlate well with the behavior of crystal cell parameters obtained by X-ray diffraction data. The temperature of the magnetic phase transition T-N = 32.54(4) K is established, below which the iron ions form a 3D magnetic order of the Izing type. The magnetic transition of the iron subsystem from a commensurate to an incommensurate structure at a temperature of about T # 15 K is discussed. The "Mossbauer " Debye temperature Theta(M) was estimated to be 485(2) K. (C) 2022 Elsevier B.V. All rights reserved.

Hydrogen bonds are one of the most enigmatic intermolecular interactions where the strength of interaction depends very intimately on the local environment of the bonded atoms. One can change the entire nature of the interactions by changing its local environment. Many of the enzymatic reactions work by this above-mentioned mechanism. Hence, it is important to understand how changes in the local environment can modify hydrogen bonds. In line with this, complete structural and vibrational investigations on ammonium ion substituted potassium dihydrogen phosphate mixed crystals K-0.988(NH4)(0.012)H2PO4 and (K-0.956(NH4)(0.044)H2PO4) are performed. The behaviour of O-H-O hydrogen bond in these crystals is very sensitive to changes in the local environment. A small change in the concentration of K+ ion has an observable effect on hydrogen bond geometry and hence on the double-well potential energy contour for these bonds. The dynamics of H atom depends on the potential barrier height. It changes gradually with decreasing temperature from predominantly inter-well to completely intra-well dynamics with an intermediate precursor region, where inter-well and intra-well dynamics are important.

A recently investigated HT-Ce2Rh2Ga was found to exhibit a magnetic phase transition at 128.5 K. A comprehensive multi-temperature investigation in the 85-400 K temperature range showed that the compound undergoes a structural phase transition as well. Upon cooling below 123.2 K its crystal symmetry changes from Cmce to C2/m being accompanied by non-merohedral twinning. Both structural transformations are reversible with a small hysteresis furthermore. The temperature of the structural phase transition was detected by anomalies in temperature dependences of unit cell dimensions, interatomic distances, and atomic displacement parameters. Negative thermal expansion along the c-axis was revealed in the range 92.7-170 K. When cooled to 200 K, Ce and Rh atoms converge to a distance sufficient for structural changes to become qualitative. Upon further cooling to 145 K, this interaction affects positions of all the atoms in the structure, leading to anisotropy of earlier equivalent interatomic distances and a sharp anomalous increase in atomic displacement parameters, the dynamics of which deviates more and more from the theoretical dependences in the Einstein and Debye approximations. (C) 2021 Elsevier B.V. All rights reserved.

Neodymium iron borate NdFe3(BO3)4 is an intensively studied multiferroic with high electric polarization values controlled by a magnetic field. It is characterized by a large quadratic magnetoelectric effect, rigidity in the base plane and a rather strong piezoelectric effect. In this work, the atomic structure of (Nd0.91Bi0.09)Fe3(BO3)4 was studied by single-crystal X-ray diffraction in the temperature range 20-500 K (space group R32, Z = 3). The Bi atoms found in the composition partially substitute the Nd atoms in the 3a position; they entered the structure due to the growth conditions in the presence of Bi2Mo3O12. It was shown that in the temperature range 20-500 K there is no structural phase transition R32→P3121, which occurs in rare-earth iron borates (RE = Eu-Er, Y) with an effective rare-earth cation radius smaller than that of Nd. The temperature dependence of the unit-cell c parameter reveals a slight increase on cooling below 90 K, which is similar to the results obtained previously for iron borates of Gd, Y and Ho. The atomic distances (Nd,Bi)-O, (Nd,Bi)-B, (Nd,Bi)-Fe, Fe-O, Fe-B and Fe-Fe in the iron chains and between chains decrease steadily with decreasing temperature from 500 to 90 K, whereas the B1(3b)-O distance does not change and the average B2(9e)-O distance increases slightly. There is a uniform decrease in the atomic displacement parameters with decreasing temperature, with a more pronounced decrease for the Nd(3a) and O2(9e) atoms. The O2(9e) atoms are characterized by the maximum atomic displacement parameters and the most elongated atomic displacement ellipsoids. The characteristic Debye and Einstein temperatures, and the static component in the atomic displacements were determined for cations using multi-temperature diffraction data. It was shown that the Nd cations have the weakest bonds with the surrounding atoms and the B cations have the strongest.

Rare-earth iron borate RFe3(BO3)4 crystals are studied worldwide lately owing to their perspective magnetoelectric and multiferroic properties [1].A major part of these single crystals was grown by flux method using Bi2Mo3O12 as a solvent [2,3].In this work temperature-dependent structural behavior of RFe3(BO3)4 (R = Ho, Y, Sm, Nd) single crystals were studied by X-ray structure analysis.The chemical composition was verified by X-ray energy-dispersive elemental analysis.Additional structure information was obtained by Mössbauer spectroscopy on 57 Fe nuclei.Bi atoms entered the composition of all the crystals during the growth process and the final compositions of single crystals studied are Ho 0.96 Bi 0.04 Fe 3 (BO 3 ) 4 , Y 0.95 Bi 0.05 Fe 3 (BO 3 ) 4 , Sm 0.93 Bi 0.07 Fe 3 (BO 3 ) 4 , and Nd 0.91 Bi 0.09 Fe 3 (BO 3 ) 4 .Unit cell parameters for R = Ho, Y, Nd were measured over 30-500 K. Parameters a,b of the crystals with R = Ho, Y are descending with temperature lowering, whereas a,b parameters of Nd-crystal do not change strongly.A sharp jump of a,b for R = Ho and Y was registered demonstrating presence of structural phase transition.At the same time, c (T) dependence has the similar character for all three crystals (R = Ho, Y, Nd) -c parameter decreases with lowering temperature to 80-100 K and then grows smoothly down to 30 K. Structure of Ho0.96Bi0.04Fe3(BO3)4,Y0.95Bi0.05Fe3(BO3)4,Sm0.93Bi0.07Fe3(BO3)4,and Nd0.91Bi0.09Fe3(BO3)4was determined at several temperatures in 90-500 K temperature range to study temperature-dependent structure peculiarities, in particular, changes during the structural phase transition for R = Ho, Y.The temperature of the phase transition Tstr = 365 К for R = Ho and Tstr = 370 К for R = Y was stated on the basis of systematic absences analysis and temperature dependence of a,b parameters.Inclusion of Bi atoms with a larger ionic radius leads to Tstr lowering in comparison with powder samples without Bi [4].The structure of the compounds with R = Ho, Y was refined in sp.gr.R32 above Tstr and in sp.gr.P3121 below it.Structure of crystals with R = Sm, Nd belongs to sp. gr.R32 at all temperatures studied.There is a slight steady decrease of specific distanced in (R,Bi)O6 trigonal prisms, FeO6 octahedra, BO3 triangles and Fe-Fe helicoidal chains with temperature lowering in sp.gr.R32.When going to lower-symmetry sp.gr.P3121 (for R = Ho, Y) and with further temperature decreasing non-uniform changes in the bond lengths are observed.Equivalent atomic displacement parameters Ueq decrease with temperature lowering.However, Ueq of oxygen atoms O1 and O2 as well as ones of boron atoms B2 and B3 (sp.gr.P3121 labels) are highly sensitive to a structural phase transition, demonstrating fluctuations around Tstr. Debye (TD) and Einstein (TE) characteristic temperatures for cations in the crystals with R = Ho, Y, Sm, Nd were calculated.Both TD and TE values are close for the same type of cations.TD and TE for R and Fe atoms in sp.gr.R32 are close to the corresponding values in sp.gr.P3121, and there is a significant change in TD, TE values for B atoms after a phase transition.Gamma-resonance measurements on 57 Fe nuclei showed that the hyperfine parameters of the Mössbauer spectra correspond to Fe 3+ ions in an octahedral oxygen environment.Quadrupole splitting Δ temperature dependence demonstrates complex behavior and is in good agreement with X-ray diffraction results.

The anisotropy of microhardness and fracture of the (0001) basal plane of langasite crystal has been studied using Berkovich indentation under a load of 100 g at different indenter positions. The formation of “crystallographic” microcracks (straight-line and emerging exactly from the impression angles), which propagate in six directions, spaced by 60°, is observed. It is suggested that the observed specific features of microcrack formation are based structurally on the presence of a mixed cation site. As follows from the structural model of a multicell, fractures propagate along the direction of polyhedra with mixed cation occupancy, and the fracture line pushes apart unit cells of different chemical compositions. The Young’s modulus of langasite crystal has been calculated for different crystallographic planes.

The conductive, magnetic, optical, and mechanical properties of the rare-earth RB12 dodecaborides (R = Sc, Y, Tb, Dy, Ho, Er, Tm, Yb, Lu) are of significant interest both for basic research and for practical applications.The combination of metal conductivity with resistance to external influences makes them unique materials for use in extreme environmental conditions.In basic research, these compounds are conveniently used to study the properties caused by rare-earth metal ions.

Synthetic tennantite, Cu12As4S13, is the analogue of an abundant mineral that belongs to the tennantite-tetrahedrite group with a low lattice thermal conductivity. Combined data from high-quality X-ray diffraction, electron microscopy (STEM-HAADF), Raman spectroscopy, as well as DFT calculations are used to analyze the peculiarities of its structure stability and dynamics. Atomic displacement parameters (ADP) and low-energy optical phonon modes are discussed within the context of the structure variations and pecularities of the charge distribution. The latter indicates that the tennantite structure tends to conform to the covalent polar bonds and its charge distribution is significantly affected by the atomic shifts in Laves polyhedra. According to the DFT calculations, there is a population of stable model structures with varying shifts of copper atoms in Laves polyhedra, with total energies being very close (within 0.24 eV), which can explain the observed behavior of the ADP. A specially designed technique is used for the experimental analysis of the ADP that revealed Einstein characteristic temperatures in the tennantite structure to be in the range of 50-190 K, which is attributed to low-energy optical phonon modes.

N.Sluchanko, 2, ∗ A.Bogach, N.Bolotina, V.Glushkov, 2 S.Demishev, 4 A.Dudka, V.Krasnorussky, O.Khrykina, K.Krasikov, V.Mironov, V.Filipov, and N.Shitsevalova Prokhorov General Physics Institute, Russian Academy of Sciences, 38 Vavilov Str., 119991 Moscow, Russia National University of Science and Technology (MISiS), 119049 Moscow, Russia Shubnikov Institute of Crystallography of Federal Scientific Research Centre Crystallography and Photonics of Russian Academy of Sciences, 59 Leninskii Ave., 119333 Moscow, Russia Moscow Institute of Physics and Technology (State University), 9 Institutskiy Per., 141700 Dolgoprudny, Russia Frantsevich Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, 3 Krzhyzhanovsky Str., 03680 Kiev, Ukraine (Dated: July 10, 2017)

Analysis of the intriguing physical properties of the dodecaborides, $R$B$_{12}$, requires accurate data on their crystal structure. We show that a simple cubic model fits well with the atomic positions in the unit cell but cannot explain the observed anisotropy in the physical properties. The cooperative Jahn-Teller (JT) effect slightly violates the ideal metric of the cubic lattice and the symmetry of the electron density distribution in the lattice interstices. Theoretical models of the JT distortions of the boron framework are presented. Their correspondence to the electron-density distribution on the maps of Fourier syntheses obtained using x-ray data and explaining the previously observed anisotropy of conductive properties is demonstrated. The effect of boron isotope composition on the character of the lattice distortions is shown. We also discuss the application of the Einstein model for cations and the Debye model for the boron atoms to describe the dynamics of the crystal lattice.