Magnetocaloric properties and unconventional critical behavior in (Gd,Tb)6(Fe,Mn)Bi2 intermetallics

The magnetic and magnetocaloric properties of the intermetallic family (Gd,Tb)6(Fe,Mn)Bi2 have been studied from 2 K to temperatures above the respective Curie temperatures TC. The substitution of Gd by Tb (Gd6FeBi2, Gd3Tb3FeBi2, Tb6FeBi2) tunes TC in the range 350-250 K and favors the apparition of a metamagnetic transition at very low temperature (below 10 K) from a complex magnetic state to a ferromagnetic one, as well as a spin reorientation transition below Tm = 72 K. As a consequence, an important inverse magnetocaloric effect (IMCE) appears below 20 K and an interesting direct magnetocaloric effect (DMCE) appears over a wide temperature span between TC and Tm with maxima at those temperatures. The partial substitution of Fe by Mn in Tb6Fe0.5Mn0.5Bi2 shifts these effects upwards in temperature while expanding the region of the direct magnetocaloric effect between 70 and 400 K. The combination of adjoint IMCE and DMCE as well as the wide span of the latter shows that tuning this family allows to locate the magnetocaloric effect in different regions of interest. The critical behavior of the PM-FM transitions has been studied obtaining the critical exponents α, β, γ, δ and checking that the respective magnetocaloric effects also scale with the critical parameters n and δ. The transition in Gd6FeBi2 belongs to the Heisenberg universality class with deviations due to magnetocrystalline anisotropies; the critical exponents for Gd3Tb3FeBi2 (in agreement with the Mean Field model) suggest the presence of long range order magnetic interactions, while Tb6FeBi2 and Tb6Fe0.5Mn0.5Bi2 present an unconventional critical behavior aligned with long range order interactions.This work has been supported by Universidad del País Vasco UPV/EHU (project GIU19/305) and the Russian Fund for Basic Research (project N° 20-03-00209-a). A. Herrero thanks the Department of Education of the Basque Government as grantee of the programme “Programa Predoctoral de Formación de Personal Investigador No Doctor”. The authors thank for technical and human support provided by SGIker of UPV/EHU

Magnetocaloric properties, magnetic interactions and critical behavior in Ho6(Fe,Mn)Bi2 intermetallics

Four polycrystaline Fe2P-type Ho6(Fe,Mn)Bi2 intermetallic compounds (space group , No. 189, hP9) have been studied using magnetic techniques in order to explore their ability as magnetocaloric materials, and study the critical behavior of the paramagnetic (PM) to ferromagnetic (FM) transitions to obtain a deeper understanding of the range of the magnetic interactions. The obtained critical exponents β, γ and δ for the four compounds studied (Ho6MnBi2, Ho6FeBi2, Ho6(Mn0.5Fe0.5)Bi2, Ho6(Mn0.75Fe0.25)Bi2) point to long-range order interactions, as they are close to those of the Mean Field Universality class. All of the compounds show relevant magnetocaloric properties over a very broad temperature range, limited by the PM-FM transition and a spin-reorientation one, well separated in all cases. They present very high values of the refrigerant capacities (from 520 J/kg to 709 J/kg at 5 T), good magnetic entropy changes (from 3.4 to 5.7 J/(kgK) at 5 T), and a flat and wide temperature span for the working temperature range (nearly 200 K for Ho6MnBi2, 80 K for Ho6FeBi2 at 5T). The change in properties with composition proves that the magnetocaloric properties can be tuned in Fe2P-type compounds to accommodate different refrigeration applications. Finally, the magnetocaloric scaling laws have been successfully tested and universal curves for the magnetic entropy change have also been obtained in the PM-FM transition region.This work has been supported by Universidad del País Vasco UPV/EHU (GIU16/93)

Polyoxometalate chemistry at volcanoes: discovery of a novel class of polyoxocuprate nanoclusters in fumarolic minerals

Polyoxometalate (POM) chemistry is an important avenue of comprehensive chemical research, due to the broad chemical, topological and structural variations of multinuclear polyoxoanions that result in advanced functionality of their derivatives. The majority of compounds in the polyoxometalate kingdom are synthesized under laboratory conditions. However, Nature has its own labs with the conditions often unconceivable to the mankind. The striking example of such a unique environment is volcanic fumaroles – the natural factories of gas-transport synthesis. We herein report on the discovery of a novel class of complex polyoxocuprates grown in the hot active fumaroles of the Tolbachik volcano at the Kamchatka Peninsula, Russia. The cuboctahedral nanoclusters {[MCu$_{12}$O$_{8}$](AsO$_{4}$)$_{8}$} are stabilized by the core Fe(III) or Ti(IV) cations residing in the unique cubic coordination. The nanoclusters are uniformly dispersed over the anion- and cation-deficient NaCl matrix. Our discovery might have promising implications for synthetic chemistry, indicating the possibility of preparation of complex polyoxocuprates by chemical vapor transport (CVT) techniques that emulate formation of minerals in high-temperature volcanic fumaroles

Tailoring the magnetocaloric, magnetic and thermal properties of Dy6(Fe,Mn)X2 intermetallics (X==Sb, Te, Bi)

[EN] The structural, magnetic, magnetocaloric (MCE) and thermal properties of seven Fe2P-type Dy6(Fe,Mn)X2 (X=Sb, Bi, Te) intermetallics (space group P 6 over line 2 m, N 189, hP9) have been experimentally studied. They present a paramagnetic to ferromagnetic transition (in the range 129-370 K), followed, as temperature decreases, by a spin-reorientation one (from 52 to 170 K) and a ground magnetic state at 2 K with anti-ferromagnetic components. This state turns into a ferromagnetic state when a magnetic field is applied. The critical exponents beta,gamma,delta related to the PM-FM transition point to long range order interactions but in most compounds their values severely deviate from the Mean Field class, presenting an unconventional critical behavior, probably due to magnetocrystalline anisotropies. This magnetic complexity has the consequence that in every intermetallic three MCE effects arise: Two direct magnetocaloric effects (DMCE) with a table-like effect in between (from 40 K to more than 400 K), with moderate values of the magnetic entropy maxima (up to 6.9 J/kgK for 140 Delta H = 5 T, with the tableau in-between being around 4 J/kgK, for Dy6FeSb2 and Dy6FeSbTe). The calculation of the Thermal Average Entropy Change allows to place the properties of two compounds (Dy6FeSb2 and Dy6FeSbTe) close to other rare earth based high entropy alloys described in literature. The seven compounds present a relevant third MCE, inverse, below 25 K, with a value as high as 17.8 J/kgK (140 Delta H = 5 T) for Dy6FeSbTe. The maximum of the magnetic entropy change at the Curie tem-perature has been shown to scale with the critical exponents found and universal curves have been built. Finally, the thermal diffusivities in the range of the DMCE have been measured, with the result that they present good values (between 1 and 3 mm2/s) to be used in real magnetocaloric refrigeration systems.This work has been supported by Universidad del Pais Vasco UPV/EHU (project GIU19/058) and the Russian Fund for Basic Research (project No 20-03-00209-a). A. Herrero thanks the Department of Education of the Basque Government as grantee of the programme "Programa Predoctoral de Formacion de Personal Investigador No Doctor". The authors thank for technical and human support provided by SGIker of UPV/EHU, specially the fruitful discussions with Dr. I. Orue

Selecting optimal R6TX2 intermetallics (R = Gd, Tb, Dy; T = Mn, Fe, Co, Ni; X = Sb, Te) for magnetic refrigeration

A complete experimental study of the physical properties playing a relevant role on magnetic refrigeration application (structural, magnetic, magnetocaloric and thermal) has been performed over nine selected Fe2P-type R6TX2 (R= Gd, Tb, Dy; T= Mn, Fe, Co, Ni; X=Sb, Te) intermetallic compounds, to work close to room temperature. Two magnetic phase transitions are present on these materials: a paramagnetic to ferromagnetic transition in the range 182-282 K and a spin reorientation transition in the range 26-76 K. As a consequence, two peaks related to a direct magnetocaloric effect (DMCE) appear in the magnetic entropy change, generating a wide table-like plateau region in between both peaks, which is required to improve the efficiency of refrigerators following an Ericsson cycle. The highest magnetic entropy peak value for μ0ΔH = 5 T is found for Tb2Dy4FeSb2, with 7.72 J/kg K around 182 K. For the same applied field the other compounds show moderate values around room temperature (2.88-4.53 J/Kg K). However, the superposition of the two peaks results in huge refrigerant capacity values, up to RCFWHM(5 T)=1103.04 J/kg in the case of Tb2Dy4FeSb2. The thermal diffusivity, effusivity conductivity and specific heat have been measured at room temperature, and the temperature dependence of the former has been obtained around the relevant magnetic phase transition region, with values in the range 1.3-2.3 mm2/s, which are good for magnetic refrigerators under high working frequencies. The study is completed with a rigorous critical behavior analisis of the second order PM-FM transition. The critical exponent γ points to long range order interactions, in general, while β values are in the range (0.59-0.90), indicating a deviation from theoretical models as a reflection of the magnetic complexity in these compounds. The critical exponents have been used to confirm the scaling relations of magnetocaloric properties, and the scaling of refrigerant capacity (RC) values in materials presenting two magnetic phase transitions is addressed, concluding that for a correct scaling of RC the magnetic entropy change peak must be considered symmetric. The role of each atom on the properties of the compounds is discussed.This work was supported by Departamento de Educación del Gobierno Vasco (project IT1430-22) and the Russian Fund for Basic Research through the project no. 20-03-00209-a, as well as by an ICDD (International Centre for Diffraction Data) (USA) grant n 05-07

Bioerosion of siliceous rocks driven by rock-boring freshwater insects

Macrobioerosion of mineral substrates in fresh water is a little-known geological process. Two examples of rock-boring bivalve molluscs were recently described from freshwater environments. To the best of our knowledge, rock-boring freshwater insects were previously unknown. Here, we report on the discovery of insect larvae boring into submerged siltstone (aleurolite) rocks in tropical Asia. These larvae belong to a new mayfly species and perform their borings using enlarged mandibles. Their traces represent a horizontally oriented, tunnel-like macroboring with two apertures. To date, only three rock-boring animals are known to occur in fresh water globally: a mayfly, a piddock, and a shipworm. All the three species originated within primarily wood-boring clades, indicating a simplified evolutionary shift from wood to hardground substrate based on a set of morphological and anatomical preadaptations evolved in wood borers (e.g., massive larval mandibular tusks in mayflies and specific body, shell, and muscle structure in bivalves)

Кристалл Er3+,Yb3+:YGdSiO5 для лазеров спектрального диапазона 1,5–1,6 мкм

Solid-state erbium lasers, emitting in the spectral range of 1.5–1.6 µm, are of great interest for several industrial applications. Nowadays the Er:glass is the most widespread laser material for obtaining laser radiation at the wavelength near 1.5 µm. However, the maximal output powers of such lasers are restricted by hundreds of milliwatts because low thermal characteristics of the glass host. By this reason the search for new crystalline hosts doped with erbium ions is the actual task.In this article the investigation results of spectroscopic properties of Er3+,Yb3+:YGdSiO5 (YGSO) crystals are reported. Polarized absorption and luminescence spectra were measured. The lifetimes of energy levels were determined. The excited state absorption spectra were measured. It was shown that excited state absorption band does not overlap with gain band in the range 1.5–1.6 µm. The energy transfer efficiency from ytterbium to erbium ions was estimated. The stimulated emission and gain cross-section spectra for Er3+ ions in YGSO were calculated.Твердотельные эрбиевые лазеры, излучающие в спектральной области 1,5–1,6 мкм, представляют большой интерес для различных практических применений. Из доступных на сегодня лазерных материалов для получения стимулированного излучения в области 1,5 мкм наибольшее распространение получили фосфатные стёкла, легированные ионами Er3+. Однако максимальные выходные мощности таких лазеров ограничены несколькими сотнями милливатт из-за низких термических характеристик стекла, поэтому поиск подходящей кристаллической матрицы для ионов эрбия является актуальным и в настоящее время.В работе представлены результаты исследований спектроскопических свойств кристалла иттрий-гадолиниевого силиката, легированного ионами эрбия и иттербия – Er3+,Yb3+:YGdSiO5 (YGSO). Измерены спектры поглощения и люминесценции в поляризованном свете, определены времена жизни возбуждённых состояний активных ионов. Измерения спектров поглощения из возбуждённого состояния показали, что полоса поглощения из возбуждённого состояния не перекрывается с полосой усиления в спектральной области 1,5–1,6 мкм. Проведена оценка эффективности переноса энергии от ионов иттербия к ионам эрбия. Выполнен расчёт спектров поперечных сечений вынужденного излучения и усиления

Crystal Chemistry of Chlormagaluminite, Mg₄Al₂(OH)₁₂Cl₂(H₂O)₂, a Natural Layered Double Hydroxide

Chlormagaluminite is the only Cl-dominated hydrotalcite-supergroup mineral species with M2+:M3+ = 2:1. The holotype sample of chlormagaluminite from the Kapaevskaya volcanic pipe (Irkutsk Oblast, Siberia, Russia) has been chemically and structurally characterized. The average chemical composition of the mineral is (electron microprobe, OH content is calculated by stoichiometry and H2O from the crystal-structure data, wt. %): MgO 33.85, FeO 1.09, Al2O3 22.07, Cl 14.72, H2Otot 30.96, Cl=O −3.39, total 99.30. The empirical formula based on Mg + Al + Fe = 6 atoms per formula unit (apfu) is [Mg3.91Fe2+0.07Al2.02(OH)12]Cl2.02(H2O)2.0(2). The crystal structure has been solved from single-crystal X-ray diffraction data in the space group P63/mcm, a = 5.268(3), c = 15.297(8) Å and V = 367.6(4) Å3. The refinement converged to R1 = 0.083 on the basis of 152 unique reflections with I > 2σ(I) collected at room conditions. The powder pattern contains standard reflections of a 2H polytype and two additional reflections [(010), d010 = 4.574 Å; (110), d110 = 2.647 Å] indicative of Mg and Al ordering according to the 3 × 3 superstructure. The structure is based upon brucite-type octahedral layers with an ordered distribution of Mg and Al over octahedral sites. The Cl− anions and H2O molecules reside in the interlayer, providing a three-dimensional integrity of the structure

Kurchatovite and Clinokurchatovite, Ideally CaMgB2O5: An Example of Modular Polymorphism

Kurchatovite and clinokurchatovite, both of ideal composition CaMgB2O5, from the type localities (Solongo, Buryatia, Russia, and Sayak-IV, Kazakhstan, respectively) have been studied using electron microprobe and single-crystal X-ray diffraction methods. The empirical formulae of the samples are Ca1.01Mg0.87Mn0.11Fe2+0.02B1.99O5 and Ca0.94Mg0.91Fe2+0.10Mn0.04B2.01O5 for kurchatovite and clinokurchatovite, respectively. The crystal structures of the two minerals are similar and based upon two-dimensional blocks arranged parallel to the c axis in kurchatovite and parallel to the a axis in clinokurchatovite. The blocks are built up from diborate B2O5 groups, and Ca2+ and Mg2+ cations in seven- and six-fold coordination, respectively. Detailed analysis of geometrical parameters of the adjacent blocks reveals that symmetrically different diborate groups have different degrees of conformation in terms of the δ angles between the planes of two BO3 triangles sharing a common O atom, featuring two discrete sets of the δ values of ca. 55° (B’ blocks) and 34° (B” blocks). The stacking of the blocks in clinokurchatovite can be presented as …(+B’)(+B”)(+B’)(+B”)… or [(+B’)(+B”)], whereas in kurchatovite it is more complex and corresponds to the sequence …(+B’)(+B”)(+B’)(−B’)(−B”)(−B’)(+B’)(+B”)(+B’)(−B’)(−B”)(−B’)… or [(+B’)(+B”)(+B’)(−B’)(−B”)(−B’)]. The B’:B” ratios for clinokurchatovite and kurchatovite are 1:1 and 2:1, respectively. According to this description, the two minerals cannot be considered as polytypes and their mutual relationship corresponds to the term modular polymorphs. From the viewpoint of information-based measures of structural complexity, clinokurchatovite (IG = 4.170 bits/atom and IG,total = 300.235 bits/cell) is structurally simpler than kurchatovite (IG = 4.755 bits/atom and IG,total = 1027.056 bits/cell). The high structural complexity of kurchatovite can be inferred from the modular character of its structure. The analysis of structural combinatorics in terms of the modular approach allows to construct the whole family of theoretically possible “kurchatovite”-type structures that bear the same structural features common for kurchatovite and clinokurchatovite. However, the crystal structures of the latter minerals are the simplest and are the only ones that have been observed in nature. The absence of other possible structures is remarkable and can be explained by either the maximum-entropy of the least-action fundamental principles