Data on vibrational spectra of the langasites Ln3CrGe3Be2O14 (Ln = La, Pr, Nd) and ab initio calculations

In “Lattice dynamics and structure of the new langasites Ln3CrGe3Be2O14 (Ln = La, Pr, Nd): vibrational spectra and ab initio calculations” [1], experimental and calculated results on lattice dynamics of the recently discovered new compounds La3CrGe3Be2O14, Pr3CrGe3Be2O14, and Nd3CrGe3Be2O14 are reported. These compounds belong to the langasite series and constitute a new class of low-dimensional antiferromagnets. The data presented in this article includes IR diffuse transmission spectra of powder samples of Ln3CrGe3Be2O14 (Ln = La, Pr, Nd) registered at room temperature with a Bruker 125HR Fourier spectrometer, Raman spectra taken in the backscattering geometry (also at room temperature) with a triple monochromator using the line 514, 5 nm of an argon laser as an excitation, results of the DFT calculations with the B3LYP and PBE0 hybrid functionals on the optimized crystal structures, eigenfrequencies and eigenvectors of the normal vibrational modes. These data can be used to analyse electron-phonon interaction and multiferroic properties of the new langasites and to compare the lattice dynamics of different langasites. The dataset is available on mendeley data public repository at https://doi.org/10.17632/32grbb4p82.1.


Data description
The dataset includes 6 text files for our measured infrared (IR) and Raman spectra of Ln 3 CrGe 3-Be 2 O 14 (Ln ¼ La, Pr, Nd, raw data) [1]. These text files are named by rare-earth (RE) element symbol plus the method used to take the spectrum, e.g., Pr_IR.txt means an IR spectrum of Pr 3 CrGe 3 Be 2 O 14 . Each Specifications Table   Subject Materials Science Specific subject area Electronic, Optical and Magnetic Materials Type of data Table  Figure Text file How data were acquired IR spectra were collected in diffuse transmission mode with Bruker 125HR Fourier spectrometer, Raman spectra were collected in the backscattering geometry with a home-made triple monochromator using the line 514, 5 nm of an argon laser as an excitation. The CRYSTAL14 program designed for simulating periodic structures in the MO LCAO approximation was used for DFT ab initio calculations. Quasi-relativistic pseudopotentials ECP46MWB, ECP59MWB, and ECP60MWB with corresponding valence basis sets ECPnMWB were taken for La, Pr, and Nd. Value of the Data These data can be used to compare the lattice dynamics of different langasites. These data can be used by researchers working on vibrational and magnetoelastic properties of langasites. These data can be used to analyse electron-phonon interaction and multiferroic properties of the new langasites.
text file has two columns which correspond to wave number (unit: cm À1 ) and IR absorbance or Raman intensity (in arbitrary units). The same data are presented also as Excel files, e.g., Pr_IR.xlsx. The data of ab initio calculations of optimized crystal structures is provided in 5 Excel tables.    [14] are shown in square brackets.  Tables 1e5, available experimental data are in square brackets. The dataset includes 3 text files for the calculated with the B3LYP hybrid functional frequencies of normal modes and their intensities in the IR and Raman spectra. These text files are named by RE element symbol plus the method to get the data, e.g., Pr_abinit.txt means the calculated data for Pr 3 CrGe 3 Be 2 O 14 . Each text file has four columns which correspond to the symmetry of the mode (irreducible representation), wave number (unit: cm À1 ), IR intensity, Raman intensity (arb. units). First, all A 1 modes are listed, they are followed by the A 2 and, then, E modes. The same data are presented also as Excel files, e.g., Pr_abinit.xlsx. Three Excel Tables, Table 6, Table 7, and Table 8, provide all calculated modes compared with those found from the measured spectra (analyzed data), in increasing order of their frequency for La 3 CrGe 3 Be 2 O 14 , Pr 3 CrGe 3 Be 2 O 14 , and Nd 3 CrGe 3 Be 2 O 14 , respectively. Mode symmetries are indicated.

Experimental design, materials, and methods
The main information on the samples and experimental equipment used to take the spectra, as well as on the calculation methods is presented in Ref. [1]. Powder samples of the studied compounds La 3 CrGe 3 Be 2 O 14 , Pr 3 CrGe 3 Be 2 O 14 , and Nd 3 CrGe 3 Be 2 O 14 were synthesized by a high-temperature solidstate reaction from high-purity La 2 O 3 , Pr 2 O 3 , Nd 2 O 3 and GeO 2 , Cr 2 O 3 (reagent grade), and BeO (99.54%). Stoichiometric amounts of oxides were thoroughly ground together, pressed into pellets, placed on a Pt substrate and sintered in air for 5 h at 1350 о С (the Nd and Pr compounds) and at 1325 о С (the La compound). To reduce the loss of GeO 2 due to evaporation, the pressed samples were encapsulated in the original powdered charges. The phase composition of sintering products was studied by X-ray diffraction using a diffractometer STOE STADI_MP in a transmission mode (CuK a1 radiation). The spасе group P321 was confirmed for all samples.
The infrared diffuse transmission and Raman scattering spectra of Ln 3 CrGe 3 Be 2 O 14 (Ln ¼ La, Pr, Nd) powder samples were measured at room temperature. Powders of Ln 3 CrGe 3 Be 2 O 14 were mixed with optical-grade KBr powder and pressed into pellets. Far-infrared diffuse transmission spectra were registered in the spectral region 50e1200 cm À1 at a resolution 2 cm À1 using a Fourier spectrometer Bruker IFS 125HR and a DTGS and a liquid-nitrogen-cooled MCT detectors. Raman spectra were taken in the backscattering geometry at a resolution 3 cm À1 with a home-made triple monochromator using the line 514, 5 nm of an argon laser as an excitation. R a m a n e a c t i v e Ab initio calculations of phonon frequencies and intensities of the infrared-and Raman-active modes of La 3 CrGe 3 Be 2 O 14 , Pr 3 CrGe 3 Be 2 O 14 , and Nd 3 CrGe 3 Be 2 O 14 were performed in a framework of the density functional theory (DFT) with the hybrid functional B3LYP [2], which takes into account both local and nonlocal (in the Hartree-Fock formalism) exchange. The sequence of calculations was as follows. The optimization of the crystal structure was carried out first. After that, the phonon spectrum was calculated for the crystal structure corresponding to the minimum energy. The CRYSTAL14 program [3] designed for simulating periodic structures in the MO LCAO approximation was used for calculations. Quasi-relativistic pseudopotentials ECP46MWB, ECP59MWB, and ECP60MWB [4,5] with corresponding valence basis sets ECPnMWB [6] were taken for La, Pr, and Nd. All-electron basis sets of TZVP type were used for Cr, Ge, Be, and O [7]. These basis sets are available at the CRYSTAL website. The reciprocal space sampling was performed by Monkhorst-Pack mesh. The algorithm of calculation of the two-electron Coulomb and exchange integrals is given in Ref. [8]. The tolerance of self-consistently solving of the system of Kohn-Sham equations was 10 À9 . The phonon spectrum was calculated in the harmonic approximation. In the Hessian matrix, the first (second) derivatives were calculated analytically (numerically). To perform numerical calculations of the second derivatives, the atom was displaced from the equilibrium position by 0.003 Å [8].
We used the Born charges when calculating Raman and infrared intensities in the CRYSTAL code [9]. Electric dipole properties were calculated using the periodic coupled-perturbed Hartree-Fock (CPHF) or Kohn-Sham (CPKS) approach [10e12].
The Plaсzek approximation was used to calculate the intensity of the Raman modes at a nonresonant excitation [11]. For an oriented single crystal, the intensity associated with the mode u k is [3]: where a k ij is an element of the Raman tensor, i; j ¼ x; y; z. The value C in (1) is defined by the laser frequency u L and the temperature T dependence as follows: C $ 1 þ nðu k Þ 30u k ðu L À u k Þ 4 ; (2) where 1 þ nðu k Þ ¼ 1 À exp À Zu k k B T À1 ; (3) nðu k Þ being the Bose occupation factor. The simulation of the intensity of Raman modes for powder sample has been done by computing integrals over all possible orientations of ideal bulk crystal. These integrals can be reduced to three rotational invariants [13]: