Phase transitions and exchange interactions in the SmCr 3 ( BO 3 ) 4 crystal

Spectroscopic investigation and specific heat and magnetic susceptibility measurements of SmCr3(BO3)4 crystals were performed. The spectra of the Sm3+ and Cr3+ ions in samarium chromium borate were calculated and parameters of the exchange interactions between the nearest chromium ions, chromium and samarium ions were determined. Three phase transitions were observed at the temperatures T1= 7.8 ± 0.5 К, T2 = 6.7 ± 0.5 К, and T3 = 4.3 ± 0.2 К, their nature is discussed. The crystal structures with different space symmetry groups R32 and C2/c were found to coexist in а SmCr3(BO3)4 single crystal. Samarium chromium borate SmCr3(BO3)4 is a member of a large family of rare-earth (RE) borates with structural type of the natural mineral huntite. These compounds are described by a general formula RM3(BO3)4, where R is Y or La-Lu and М is Al, Ga, Fe, Cr, or Sc. The compounds are actively investigated due to their interesting physical properties and a promising application potential. However, RE chromium borates are not so well studied. It is known that NdCr3(BO3)4 is an easy-plane antiferromagnet below the temperature 8.0K [1] and that EuCr3(BO3)4 undergoes an antiferromagnetic phase transition at TN ≈ 9 К [2]. A phase transition at 5 К was supposed to occur in SmCr3(BO3)4 [3]. We succeeded to prepare a big oriented single crystal of SmCr3(BO3)4 and to perform magnetic measurements on it. It follows from the temperature dependences of the magnetic susceptibility that the magnetic moments of chromium and samarium ions (samarium ions are polarized by the exchange field) are ordered in the ab-plane. However, as this sample was not preserved, heat capacity and optical spectra measurements were carried out on other samples. Crystal characterization by X-ray diffraction and infrared (IR) spectroscopy revealed a polytypic nature of SmCr3(BO3)4. The crystals can be rhombohedral (space group R32) or monoclinic (C2/c), depending on growth conditions. Moreover, domains of both structural phases can coexist in one single crystal. For measurements we chose the crystals with a predominant content of the R32 phase. The temperature dependence of the * Corresponding author: elenadobretsova89@gmail.com DOI: 10.1051/ , 0 (2017) 713202008 EPJ Web of Conferences epjconf/201 SPECTROSCOPY.SU 2016 132 2008 © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/). heat capacity C(T) of SmCr3(BO3)4 has revealed two second-order phase transitions, at T1 = 7.8 К and T2 = 6.7 К, and the first-order transition at T3 = 4.3 К. Optical spectra registered in the range of f-f transitions in the Sm3+ ions and d-d transitions in the Cr3+ ions helped to identify energy level positions in these ions in the crystal field (CF) of SmCr3(BO3)4. It was also found from the observed spectral lines splitting that the compound orders magnetically at a temperature near 7 К. The set of experimental data allowed determining CF parameters for the Sm3+ and Cr3+ ions in SmCr3(BO3)4. The g-factors, calculated in this paper for the ground orbital singlet of Cr3+, are gaa=1.9648, gbb=1.9675, gcc=1.9702 but the initial CF splitting parameter is D = 0.379 cm-1. When considering magnetic properties of SmCr3(BO3)4, we supposed the presence of isotropic exchange interactions between the nearest neighbor chromium and samarium ions ( 2 fd fd Cr Sm H J S S ), chromium ions inside the chains ( 12 1 2 2 nn Cr Cr H J S S ), and the next nearest neighbor chromium ions in the closest chains ( 12 1 2 ' 2 nnn Cr Cr H J S S ). A projection of the exchange interaction operator of the Sm3+ ion with one of the closest Cr3+ ions onto the ground state (the lowest doublet) of the samarium ion in the crystal-field leads to a spin Hamiltonian ( ) , , || , 2 [ ( ) ] eff fd fd Cr x x Cr y y Cr z z H J G S S S S G S S , (1) where S are components of the effective spin of the samarium ion S=1/2, and G||=0.2743, G =2.2. Temperature dependences of the static magnetic susceptibility tensor components and magnetic-field dependences of the magnetization were calculated in the self-consistent field approximation, using a model of clusters (dimers) containing the closest chromium ions in a chain. This model is analogous to the model of dimers in the chains of iron ions, used formerly to investigate magnetic and spectral properties of the RE iron borates [4]. A comparison between the calculated and experimental temperature dependences of the magnetic susceptibility gave the values Jnn=-2.8 K and Jnnn=0.75 K for the exchange integrals; the value Jfd=0.132 K was obtained from the measured samarium ion ground-state splitting in the exchange field at low temperatures on the assumption of a collinear antiferromagnetic structure in the chains of chromium ions. It follows from the analysis of magnetic characteristics of Sm3+ and Cr3+ that a hypothesis about a spin-reorientational nature of the low-temperature transition with a rotation of magnetic moments from the ab plane to the с axis is not valid. The main question about the exchange interaction parameters in comparison with iron borates refers to the exchange interaction between the chromium chains. According to neutron scattering data, the iron ions are ferromagnetically ordered in every ab plane, due to the antiferromagnetic interaction (Jnnn <0) of the closest chains. Because of the change of sign in the exchange integral Jnnn in SmCr3(BO3)4, one may expect a magnetic structure essentially different from the one observed in iron borates. Additional investigation is required to determine a magnetic structure of chromium borates. This work was supported by the Russian Science Foundation (Grant No 14-12-01033). E.A.D. acknowledges a Scholarship of the President of the Russian Federation (SP754.2015.1).

Samarium chromium borate SmCr3(BO3)4 is a member of a large family of rare-earth (RE) borates with structural type of the natural mineral huntite.These compounds are described by a general formula RM3(BO3) 4, where R is Y or La-Lu and М is Al, Ga, Fe, Cr, or Sc.The compounds are actively investigated due to their interesting physical properties and a promising application potential.However, RE chromium borates are not so well studied.It is known that NdCr3(BO3)4 is an easy-plane antiferromagnet below the temperature 8.0K [1] and that EuCr3(BO3)4 undergoes an antiferromagnetic phase transition at TN ≈ 9 К [2].A phase transition at 5 К was supposed to occur in SmCr3(BO3)4 [3].
We succeeded to prepare a big oriented single crystal of SmCr3(BO3)4 and to perform magnetic measurements on it.It follows from the temperature dependences of the magnetic susceptibility that the magnetic moments of chromium and samarium ions (samarium ions are polarized by the exchange field) are ordered in the ab-plane.However, as this sample was not preserved, heat capacity and optical spectra measurements were carried out on other samples.Crystal characterization by X-ray diffraction and infrared (IR) spectroscopy revealed a polytypic nature of SmCr3(BO3)4.The crystals can be rhombohedral (space group R32) or monoclinic (C2/c), depending on growth conditions.Moreover, domains of both structural phases can coexist in one single crystal.For measurements we chose the crystals with a predominant content of the R32 phase.The temperature dependence of the heat capacity C(T) of SmCr3(BO3)4 has revealed two second-order phase transitions, at T1 = 7.8 К and T2 = 6.7 К, and the first-order transition at T3 = 4.3 К.
Optical spectra registered in the range of f-f transitions in the Sm 3+ ions and d-d transitions in the Cr 3+ ions helped to identify energy level positions in these ions in the crystal field (CF) of SmCr3(BO3)4.It was also found from the observed spectral lines splitting that the compound orders magnetically at a temperature near 7 К.
The set of experimental data allowed determining CF parameters for the Sm 3+ and Cr 3+ ions in SmCr3(BO3)4.The g-factors, calculated in this paper for the ground orbital singlet of Cr 3+ , are gaa=1.9648,gbb=1.9675,gcc=1.9702but the initial CF splitting parameter is D = -0.379cm -1 .When considering magnetic properties of SmCr3(BO3)4, we supposed the presence of isotropic exchange interactions between the nearest neighbor chromium and samarium ions ( 2

H J G S S S S GS S
A , where SD are components of the effective spin of the samarium ion S=1/2, and G||=0.2743,GA=2.2.
Temperature dependences of the static magnetic susceptibility tensor components and magnetic-field dependences of the magnetization were calculated in the self-consistent field approximation, using a model of clusters (dimers) containing the closest chromium ions in a chain.This model is analogous to the model of dimers in the chains of iron ions, used formerly to investigate magnetic and spectral properties of the RE iron borates [4].A comparison between the calculated and experimental temperature dependences of the magnetic susceptibility gave the values Jnn=-2.8K and Jnnn=0.75K for the exchange integrals; the value Jfd=0.132K was obtained from the measured samarium ion ground-state splitting in the exchange field at low temperatures on the assumption of a collinear antiferromagnetic structure in the chains of chromium ions.
It follows from the analysis of magnetic characteristics of Sm 3+ and Cr 3+ that a hypothesis about a spin-reorientational nature of the low-temperature transition with a rotation of magnetic moments from the ab plane to the с axis is not valid.The main question about the exchange interaction parameters in comparison with iron borates refers to the exchange interaction between the chromium chains.According to neutron scattering data, the iron ions are ferromagnetically ordered in every ab plane, due to the antiferromagnetic interaction (Jnnn <0) of the closest chains.Because of the change of sign in the exchange integral Jnnn in SmCr3(BO3)4, one may expect a magnetic structure essentially different from the one observed in iron borates.Additional investigation is required to determine a magnetic structure of chromium borates.
This work was supported by the Russian Science Foundation (Grant No 14-12-01033).E.A.D. acknowledges a Scholarship of the President of the Russian Federation (SP-754.2015.1).
the exchange interaction operator of the Sm 3+ ion with one of the closest Cr 3+ ions onto the ground state (the lowest doublet) of the samarium ion in the crystal-field leads to a spin Hamiltonian