A wide series of substitutions in a subsystem of small radii cations M = Al, Cr, Fe, Ga, Sc is possible in rare-earth borates ReM3(BO3)4 with the huntite structure. However, the existence of these compounds where M = Mn still raises doubts. The study presents a synthetic approach developed in the framework of the flux method allowing for the required conditions for introducing Mn into the huntite structure and growing single crystals from the GdFe3-& khcy;Mnx(BO3)4 solid solution. Special attention is paid to the choice of the solvent and elaboration of intermediate chemical bonds formed between the components of the solvent and crystal-forming part during synthesis. The explored flux composition based on Bi2O3-B2O3-Na2O (with the 1: 23.3: 2.66 molar ratio) allows obtaining isometric dark-brown crystals of GdFe3-& khcy;Mnx(BO3)4 slightly elongated along the c axis, with x = 0.27 and the size up to 5 & times; 4 & times; 4 mm3. The X-ray powder analysis confirms the structure to be identical to that of pure GdFe3(BO3)4. Notably, their lattice parameters change despite the equivalence of the ionic radii of Fe3+ and Mn3+ cations. The analysis of the thermal and field magnetization dependences of the synthesized GdFe3-& khcy;Mnx(BO3)4 reveals a series of low-temperature features in the form of a shift of the temperatures TN and TSR as compared to those characteristic for unsubstituted GdFe3(BO3)4 and a new feature is observed to appearat T1 = 16 K to a cascade of phase transitions due to the presence of manganese in the structure.

In the substituted Nd1-xTbxFe3(BO3)4 (x = 0.1 and x = 0.2), possessing almost easy -axis magnetic structure at low temperatures, an unusual two-step transition in fields along the trigonal c axis was observed by magnetization and single -crystal neutron diffraction studies. At the first step, only part of the Tb Ising-type moments flip to the c axis, which is accompanied by a significant deviation of the antiferromagnetic Fe spins from the c axis. At the second step, the remaining Tb moments flip and the Fe moments flop into the basal plane. The observed evolution is qualitatively explained by a model assuming small deviations of Tb moments from the trigonal axis due to local environment distortions, which leads to nonequivalence of the Tb ions with respect to effective Tb-Fe exchange and external field. Thus, an intrinsic "hidden" instability of the magnetic system in the magnetic field occurs.

Experimental and theoretical study of submillimeter (terahertz) spectroscopic and magnetic properties of the rare-earth aluminum borate HoAl3(BO3)4 were performed at temperatures 3–300K. In the transmittance spectra a number of resonance lines were detected at frequencies 2–35 cm–1 for different radiation polarizations. These modes were identified as transitions between the lower levels of the ground multiplet of the Ho3+ ion split by the crystal field, including both transitions from the ground state to the excited ones and transitions between the excited states. The established excitation conditions of the observed modes and the simulation of the spectra made it possible to separate the magnetic and electric dipole transitions and to determine the energies of the corresponding states, their symmetry, and the matrix elements of the transitions. Low-frequency lines that do not fit into the established picture of the electron states of Ho3+ were also found; these lines, apparently, correspond to the ions with the distorted by defects local symmetry of the crystal field.

Absorption and magnetic circular dichroism (MCD) spectra have been studied in the region of 5 I 8 -> 5 F 2 , 5 S 2 , 5 F 3 , and 5 F 5 absorption bands of the Ho3+ ion and in the region of 4 I 9/2 -> 4 G 7/2 , 2 K 13/2 , 4 S 3/2 , 4 F 7/2 , 4 F 5/2 and 2 H 9/2 absorption bands of the Nd3+ ion in an antiferromagnetic trigonal crystal Ho 0.75 Nd 0.25 Fe 3 (BO 3 ) 4 in the temperature range from 5 to 90 K. A qualitative change in the shape of the MCD spectra of Ho3+ ion was revealed during the spin-reorientation transition at T R = 6.9 & Kcy;. Below T R , in the easy-axis state of the crystal, the MCD spectrum has a shape typical for paramagnets, i.e., it consists of diamagnetic and paramagnetic parts. Just above T R , in the easy-plane state of the crystal, MCD of Ho3+ ion has a spectrum similar to the absorption spectrum, which is typical for the paramagnetic part of the MCD. It was shown, that in the easy-plane state the exchange field of iron, being perpendicular to the external field directed along the C 3 axis, quenches the usual paramagnetic and diamagnetic MCD in Ho3+ ion, but creates a condition for the appearance of a large paramagnetic MCD of mixing (B-term). With temperature increasing, the MCD spectrum of Ho3+ ion gradually turns into a spectrum typical for paramagnets with the predominance of the sign-changing diamagnetic part, because the influence of the exchange field decreases. The shape of the MCD spectra of Nd3+ ions are typical for paramagnets but does not change during the reorientation transition in the rest of the crystal. This means that the magnetic moment of the Nd3+ ion does not make a reorientation transition simultaneously with the Fe3+ and Ho3+ ions of the rest of the crystal. This phenomenon is accounted for by a strong local magnetic anisotropy of the Nd3+ ion and by a weak exchange interaction Fe-Nd, which is not enough for rotation of the Nd3+ ion moment synchronously with that of the Fe3+ ion. The magneto-optical properties of neodymium are controlled by the easy- axis anisotropy of neodymium.

The crystal structures and hyperfine magnetic parameters of EuFe3(BO3)4 and mixed Eu0.82La0.18Fe3(BO3)4 were studied over a wide temperature range in order to analyze correlations of the structural and magnetic features and the phase transitions in multiferroic compounds of the rare-earth iron borate family. The chemical compositions of the crystals are reported from X-ray fluorescence analysis. The crystal structures of EuFe3(BO3)4 and Eu0.82La0.18Fe3(BO3)4 were determined using single-crystal X-ray diffraction in the temperature range 25-500 K. A structural phase transition is observed in EuFe3(BO3)4 below 89 K which is related to distortions in the interatomic distances and angles. The most significant of which are for R-O, R-B, R-Fe, Fe-O and Fe-Fe distances, and the angles between the BO3 triangles and the ab plane. There is no structural phase transition in lanthanum-doped EuFe3(BO3)4 based on specific heat measurements (2.2-101.3 K) and structure analysis (25-500 K), and the temperature dependences of the interatomic distances and angles are smooth. The lengths of the superexchange paths needed for the appearance of a structural phase transition in RFe3(BO3)4 have been proposed. Negative thermal expansion is observed for both compounds below 90 K, resulting from a growth of the interatomic Fe-Fe distances in the iron chains during cooling. The largest atomic displacement parameters are observed for O atoms (O2), indicating that they have the highest mobility. The magnetic properties of EuFe3(BO3)4 and Eu0.82La0.18Fe3(BO3)4 were analyzed using Mössbauer spectroscopy in the temperature range 4.5-298 K. Néel temperatures (TN) of 34.57 (1) and 32.22 (1) K are obtained based on Mössbauer spectroscopy for the pure and doped crystals, respectively. The maximum specific heat capacity temperature dependence related to the magnetic phase transition for the doped crystal is observed at 31.2 K. A violation of the strict arrangement of antiferromagnetic ordering in the ab plane in the La-doped crystals at low temperatures is suggested. The magnetic contributions of the two structural positions of the iron ions to the Mössbauer spectra could not be distinguished in either pure and doped compounds, regardless of whether they are in the paramagnetic and antiferromagnetic regions.

The polarized absorption spectra of the HoAl3(BO3)4 3 (BO 3 ) 4 single crystal were studied in the region of f - f transitions 5I8 -> 5F5, I 8 -> 5 F 5 , 5 S 2 + 5 F 4 , 5 F 3 and 5 F 2 inside the 4f f shell of the holmium ion as a function of temperature in the range of 5-90 K. In a number of absorption lines the splitting which are not provided by the local point group D 3 symmetry of holmium ion in the crystal are found. This phenomenon is explained by local changes in the structure in both the excited and ground states of holmium under the influence of a perturbation caused by a photon. This perturbation mixes the ground and excited states and changes the interaction of the atom with its environment. It was also shown that the magnitude of the splitting depends on the polarization of the incident light and on the temperature of the sample.

The phase formation of terbium chromoborate TbCr3(BO3)4 in the bismuth trimolybdate and lithium tungstate melt-solutions has been studied. The absence of the terbium chromoborate trigonal phase in the bismuth trimolybdate-based system at all component ratios has been shown. The component ratio in the lithium tungstate-based system has been found at which the TbCr3(BO3)4 trigonal crystals are formed at temperatures above 1100 degrees C; below this temperature, the monoclinic phase dominates. The structural and magnetic properties of the grown crystals have been studied. It has been established that the trigonal and monoclinic TbCr3(BO3)4 crystals synthesized from the lithium tungstate-based solvent exhibit identical magnetic properties. At the same time, a significant difference of the magnetic properties of the single crystals synthesized from the bismuth molybdate melt-solution has been observed. This difference has been attributed to the effect of Bi3+ ions that partially replace Tb3+ ions.

We present an extensive study of the luminescence characteristics of Mn impurity ions in a YAl3(BO3)4:Mn crystal, in combination with X-ray fluorescence analysis and determination of the valence state of Mn by XANES (X-ray absorption near-edge structure) spectroscopy. The valences of manganese Mn2+(d5) and Mn3+(d4) were determined by the XANES and high-resolution optical spectroscopy methods shown to be complementary. We observe the R1 and R2 luminescence and absorption lines characteristic of the 2E ↔ 4A2 transitions in d3 ions (such as Mn4+ and Cr3+) and show that they arise due to uncontrolled admixture of Cr3+ ions. A broad luminescent band in the green part of the spectrum is attributed to transitions in Mn2+. Narrow zero-phonon infrared luminescence lines near 1060 nm (9400 cm−1) and 760 nm (13,160 cm−1) are associated with spin-forbidden transitions in Mn3+: 1T2 → 3T1 (between excited triplets) and 1T2 → 5E (to the ground state). Spin-allowed 5T2 → 5E Mn3+ transitions show up as a broad band in the orange region of the spectrum. Using the data of optical spectroscopy and Tanabe–Sugano diagrams we estimated the crystal-field parameter Dq and Racah parameter B for Mn3+ in YAB:Mn as Dq = 1785 cm−1 and B = 800 cm−1. Our work can serve as a basis for further study of YAB:Mn for the purposes of luminescent thermometry, as well as other applications.

GdFe 3 (BO 3 ) 4 , single crystals were grown from melt-solutions based on bismuth trimolybdate and lithium tungstate. Single crystals of gadolinium ferroborate from lithium-tungstate solution-melt were grown for the first time. The magnetic properties of the grown crystals are compared. It is shown that the GdFe 3 (BO 3 ) 4 , ferroborate obtained using bismuth solution-melt trimolybdate contains impurities of Bi 3+ ions (6% at.), which replace Gd 3+ ions. Whereas GdFe 3 (BO 3 ) 4 , ferroborate grown from a solution-melt based on lithium tungstate does not seem to contain such uncontrolled impurities. Keywords: crystal growth, antiferromagnets, multferroics.

Cu 2 FeBO 5 ludwigite single crystals have been grown from a solution–melt by spontaneous crystallization. Using the X-ray diffraction method, the crystal structure has been resolved in detail. Cations in sites M2, M3, and M4 have turned out to be structurally disordered. It has been found that oxygen atoms are disordered in one of five nonequivalent sites (O4). As can be seen from Mössbauer spectroscopy data, Fe 3+ ions occupy four nonequivalent sites with different distortions of coordination octahedra. In the temperature range 40 K ≤ T ≤ 300 K, the spectra represent a superposition of quadrupole doublets. Static susceptibility measurements have revealed two magnetic features at T 1 = 35 K and T 2 = 20 K and spin-glass effects. Specific heat measurements in the interval 4–300 K have not discovered magnetic-transition-related anomalies.

Experimental and theoretical study of submillimeter (terahertz) spectroscopic and magnetic properties of the rare-earth aluminum borate HoAl 3 (BO 3 ) 4 were performed at temperatures 3–300K. In the transmittance spectra a number of resonance lines were detected at frequencies 2–35 cm –1 for different radiation polarizations. These modes were identified as transitions between the lower levels of the ground multiplet of the Ho 3+ ion split by the crystal field, including both transitions from the ground state to the excited ones and transitions between the excited states. The established excitation conditions of the observed modes and the simulation of the spectra made it possible to separate the magnetic and electric dipole transitions and to determine the energies of the corresponding states, their symmetry, and the matrix elements of the transitions. Low-frequency lines that do not fit into the established picture of the electron states of Ho 3+ were also found; these lines, apparently, correspond to the ions with the distorted by defects local symmetry of the crystal field.

Iron borates NdFe3(BO3)4 and SmFe3 (BO3)4 activated with 1

The Raman spectra of four crystals of TbFe3–xGax(BO3)4 solid solutions (x from 0 to 0.54) were studied in the temperature range from 8 to 350 K. The temperatures of structural phase transitions were determined. The observed spectral behavior is characteristic to condensation and restoration of soft modes. Soft modes are associated with a structural phase transition from the R32 phase to the P3121 phase. The Compositions-Temperature phase diagram was constructed.

The high-resolution optical absorption spectra of NdFe3(BO3)4 single crystals have been recorded at temperatures from 4 to 40 K in the IR range of f–f transitions in a Nd3+ ion. Light linearly polarized at different angles to the C2 axes in the basal plane has been passed along the trigonal C3 axis. Below the temperature of magnetic moment ordering into a collinear antiferromagnetic structure (TN ≈ 30 K), dichroism, that is, the absorption versus polarization dependence, arises. The temperature and angular dependences of dichroism indicate that the magnetic moments of iron are directed along the C2 axes up to about 17 K, the number of domains with variously directed C2 axes being different. The mechanism of linear dichroism has been discussed. Below 17 K, a smooth transition to the helicoidal magnetic phase has been observed, with the collinear phase coexisting with the helicoidal one. Data presented in this article contradict the earlier concept of magnetic moments fluctuating in the low-temperature phase near the C2 axis within the ±10° interval.

Polarized absorption spectra of crystal Ho0.75Nd0.25Fe3(BO3)4 in the region of the 5I8 -> 5F5 absorption band of Ho ions were investigated depending on temperature in the range of 5-20 K. Absorption spectra are decomposed into components of the Lorentz form. The spin-reorientation phase transition was detected at 6.9 K. The observed line position shifts during the reorientation transition were different for different lines, which indicated different local magnetic anisotropy of the Ho ion in its excited states. Significant changes in absorption line intensities as a function of temperature were observed during the reorientation transition and above. These were explained as a consequence of the change in even components of the local crystal field in excited states. These components mix the closely spaced 4f states. Distortion of local symmetry in the ground state was detected at one of the electron transitions.

The phase formation of terbium chromium borate in melt solutions based on bismuth trimolybdate and lithium tungstate was studied. It was shown that there is no trigonal phase of terbium chromium borate in a system based on bismuth trimolybdate at all component ratios. The ratio of components of a system based on lithium tungstate has been found, at which trigonal TbCr_3(BO_3)_4 crystals are formed at temperatures above 1100^∘C, and below this temperature only a monoclinic phase is formed. The X-ray properties of the grown crystals were studied.

Монокристаллы GdFe3(BO3)4 выращены из растворов-расплавов на основе тримолибдата висмута и вольфрамата лития. Монокристаллы ферробората гадолиния из литиево-вольфраматного раствор-расплава были выращены впервые. Проведено сравнение магнитных свойств выращенных кристаллов. Показано, что в ферроборате GdFe3(BO3)4, полученном с использованием тримолибдат висмутового раствор-расплава, присутствуют примеси ионов Bi3+ (6% at.), которые замещают ионы Gd3+. Тогда как ферроборат GdFe3(BO3)4, выращенном из раствор-расплава на основе вольфрамата лития, по-видимому, подобных неконтролируемых примесей не содержит. Ключевые слова: рост кристаллов, антиферромагнетики, мультферроики.

Structural features of new mixed bismuth-containing samarium iron–aluminium borate single crystals Sm1−xBixFe3−yAly(BO3)4 (x = 0.05–0.07, y = 0–0.28) were studied using X-ray diffraction analysis based on aluminium content and temperature in the range 25–500 K. The crystals were grown using the solution-in-melt technique with Bi2Mo3O12 in a flux. The composition of the single crystals was analyzed using energy-dispersive X-ray fluorescence and energy-dispersive X-ray elemental analysis. Temperature dependencies of Sm1−xBixFe3−yAly(BO3)4 unit-cell parameters were studied. Negative thermal expansion was identified below 100 K and represented by characteristic surfaces of the thermal expansion tensor. (Sm,Bi)–O, (Sm,Bi)–(Fe,Al), (Fe,Al)–(Fe,Al), and (Fe,Al)–O interatomic distances decreased with the addition of aluminium atoms. An increase in the (Fe,Al)–(Fe,Al) intrachain bond length at low temperatures in the magnetically ordered state weakened this bond, whereas a decrease in the (Fe,Al)–(Fe,Al) interchain distance strengthened super-exchange paths between different chains. It was found that the addition of aluminium atoms influenced interatomic distances in Sm1−xBixFe3−yAly(BO3)4 much more than lowering the temperature from 293 K to 25 K. The effect of aluminium doping on magnetoelectric properties and structural symmetry of rare-earth iron borates is also discussed.

Experimental and theoretical study of submillimeter (terahertz) spectroscopic and magnetic properties of the rare-earth aluminum borate HoAl3(BO3)4 were performed at temperatures 3–300 K. In the transmittance spectra a number of resonance lines were detected at frequencies 2–35 cm–1 for different radiation polarizations. These modes were identified as transitions between the lower levels of the ground multiplet of the Ho3+ ion split by the crystal field, including both transitions from the ground state to the excited ones and transitions between the excited states. The established excitation conditions of the observed modes and the simulation of the spectra made it possible to separate the magnetic and electric dipole transitions and to determine the energies of the corresponding states, their symmetry, and the matrix elements of the transitions. Low-frequency lines that do not fit into the established picture of the electron states of Ho3+ were also found; these lines, apparently, correspond to the ions with the distorted by defects local symmetry of the crystal field.