The single crystal of SnBi4Te4—a member of n(Bi2)∙m(SnBi2Te4) homological series and potential candidate material with nontrivial topology was grown by the modified Bridgman method and characterized by X-ray diffraction (XRD), scanning electron microscopy with an energy dispersive X-ray spectrometer (SEM–EDX), and X-ray photoelectron spectroscopy (XPS). High single-crystalline quality of the grown crystal without any signs of surface contaminants detectable by SEM–EDX and XPS was confirmed by XRD. According to the results of Rietveld analysis of the XRD data, a trigonal structure with the P-3m1 space group symmetry and a stacking sequence of atomic layers similar to that of the 9P polytype crystal structure of GeSb4Te4 is inherent to SnBi4Te4. The crystal structure is built up from 7-layer rocksalt-type slabs, alternated with bismuth bilayers, with van der Waals (vdW) bonding between the two. Raman spectroscopy and spectroscopic ellipsometry (SE), complemented and supported by ab-initio calculations, provided the core information regarding the lattice dynamics and dielectric function of SnBi4Te4, as well as preliminary insight into possible functionalities of the grown material.

Phase equilibria in the MnSe-Ga2Se3-In2Se3 system were studied by differential thermal analysis, X-ray diffraction analysis, and scanning electron microscopy techniques. A number of polythermal and isothermal sections of the phase diagram, as well as a projection of the liquidus surface were constructed. The fields of primary crystallization of seven phases, as well as, the types and coordinates of in-and monovariant equilibria in the system were determined. It was shown that wide solid solution areas formed on the MnGa2Se4-MnIn2Se4 section expand significantly towards the boundary system Ga2Se3-In2Se3 and result in large homogeneous areas. The homogeneity regions based on Ga2Se3 extend significantly-over 40 mol%-into the composition triangle, forming a broad homogeneity area. As part of this study, the boundary quasi-binary system MnSe-In2Se3 was reinvestigated, and a new phase diagram, markedly different from that reported in the literature, was constructed. It was found that the system forms a congruently melting MnIn2Se4 compound at 1193 K and an incongruently melting Mn2In2Se5 compound at 1196 K via a peritectic reaction. MnIn2Se4 has a wide homogeneity area. Based on the powder diffraction patterns, the crystal structures of both compounds were determined and their lattice parameters were refined by the Rietveld method. The study of the temperature dependence of electrical conductivity and the volt-ampere characteristics of the MnGaInSe4 compound revealed that the current flow in the nonlinear region of the current-voltage curve corresponds to Frenkel's theory of thermoelectric ionization. The concentration of ionized centers in this crystal (Nt =9 & sdot;1014 cm-3), the mean free path of charge carriers (lambda 1= 7.3 & sdot;10-5 cm) and the shape of the potential well of traps were determined. The activation energy of the charge carriers was determined from the temperature dependence of the electrical conductivity of the MnGaInSe4 crystal (E = 0.30 eV).

Complex chalcogenides based on transition elements, in particular ternary compounds of the АВ2Х4 type (M = Mn, Fe, Co, Ni; B = Ga, In, Sb, Bi; X = S, Se, Te) are among the important functional materials. Compounds of this class exhibit the phenomena of electronically or optically controlled magnetism and are very promising for the creation of lasers, light modulators, photodetectors, and other functional devices controlled by a magnetic field. Recent studies demonstrated that these compounds can also find application in photocatalysis, photovoltaics, and thermoelectric converters. The study presents new data on phase equilibria in the MnSe–In2Se3 system, obtained by differential thermal analysis, X-ray phase analysis, and scanning electron microscopy. Two ternary compounds, MnIn2Se4 with congruent melting at 1193 K and Mn2In2Se5, melting incongruently at 1196 K, were formed in the system. The first is a phase of variable composition and has a 5–6 mol. % homogeneity region towards an excess of In2Se3. Based on powder diffraction data, the Rietveld method was used to refine the crystal structures and lattice parameters of both ternary compounds

Magnetic topological insulators (TIs) herald a wealth of applications in spin-based technologies, relying on the novel quantum phenomena provided by their topological properties. Particularly promising is the (MnBi$_2$Te$_4$)(Bi$_2$Te$_3$)$_n$ layered family of established intrinsic magnetic TIs that can flexibly realize various magnetic orders and topological states. High tunability of this material platform is enabled by manganese-pnictogen intermixing, whose amounts and distribution patterns are controlled by synthetic conditions. Positive implication of the strong intermixing in MnSb$_2$Te$_4$ is the interlayer exchange coupling switching from antiferromagnetic to ferromagnetic, and the increasing magnetic critical temperature. On the other side, intermixing also implies atomic disorder which may be detrimental for applications. Here, we employ nuclear magnetic resonance and muon spin spectroscopy, sensitive local probe techniques, to scrutinize the impact of the intermixing on the magnetic properties of (MnBi$_2$Te$_4$)(Bi$_2$Te$_3$)$_n$ and MnSb$_2$Te$_4$. Our measurements not only confirm the opposite alignment between the Mn magnetic moments on native sites and antisites in the ground state of MnSb$_2$Te$_4$, but for the first time directly show the same alignment in (MnBi$_2$Te$_4$)(Bi$_2$Te$_3$)$_n$ with n = 0, 1 and 2. Moreover, for all compounds, we find the static magnetic moment of the Mn antisite sublattice to disappear well below the intrinsic magnetic transition temperature, leaving a homogeneous magnetic structure undisturbed by the intermixing. Our findings provide a microscopic understanding of the crucial role played by Mn-Bi intermixing in (MnBi$_2$Te$_4$)(Bi$_2$Te$_3$)$_n$ and offer pathways to optimizing the magnetic gap in its surface states.

AbstractMagnetic topological insulators (TIs) herald a wealth of applications in spin‐based technologies, relying on the novel quantum phenomena provided by their topological properties. Particularly promising is the (MnBi2Te4)(Bi2Te3)n layered family of established intrinsic magnetic TIs that can flexibly realize various magnetic orders and topological states. High tunability of this material platform is enabled by manganese–pnictogen intermixing, whose amounts and distribution patterns are controlled by synthetic conditions. Here, nuclear magnetic resonance and muon spin spectroscopy, sensitive local probe techniques, are employed to scrutinize the impact of the intermixing on the magnetic properties of (MnBi2Te4)(Bi2Te3)n and MnSb2Te4. The measurements not only confirm the opposite alignment between the Mn magnetic moments on native sites and antisites in the ground state of MnSb2Te4, but for the first time directly show the same alignment in (MnBi2Te4)(Bi2Te3)n with n = 0, 1 and 2. Moreover, for all compounds, the static magnetic moment of the Mn antisite sublattice is found to disappear well below the intrinsic magnetic transition temperature, leaving a homogeneous magnetic structure undisturbed by the intermixing. The findings provide a microscopic understanding of the crucial role played by Mn–Bi intermixing in (MnBi2Te4)(Bi2Te3)n and offer pathways to optimizing the magnetic gap in its surface states.

Phase equilibria in the MnSe–Ga2Se3 system were re-investigated by differential thermal analysis (DTA) and X-ray powder diffraction (XRD), and a phase diagram was constructed, somewhat differing from that presented earlier in the literature. We found that the system featured the formation of an intermediate phase (γ) with a wide (47–61 mol

A single crystalline layered semiconductor In1.2Ga0.8S3 phase was grown, and by intercalating p-aminopyridine (NH2-C5H4N or p-AP) molecules into this crystal, a new intercalation compound, In1.2Ga0.8S3·0.5(NH2-C5H4N), was synthesized. Further, by substituting p-AP molecules with p-ethylenediamine (NH2-CH2-CH2-NH2 or p-EDA) in this intercalation compound, another new intercalated compound—In1.2Ga0.8S3·0.5(NH2-CH2-CH2-NH2) was synthesized. It was found that the single crystallinity of the initial In1.2Ga0.8S3 samples was retained after their intercalation despite a strong deterioration in quality. The thermal peculiarities of both the intercalation and deintercalation of the title crystal were determined. Furthermore, the unit cell parameters of the intercalation compounds were determined from X-ray diffraction data (XRD). It was found that increasing the c parameter corresponded to the dimension of the intercalated molecule. In addition to the intercalation phases’ experimental characterization, the lattice dynamical properties and the electronic and bonding features of the stoichiometric GaInS3 were calculated using the Density Functional Theory within the Generalized Gradient Approximations (DFT-GGA). Nine Raman-active modes were observed and identified for this compound. The electronic gap was found to be an indirect one and the topological analysis of the electron density revealed that the interlayer bonding is rather weak, thus enabling the intercalation of organic molecules.

The phase relationship in the SnTe-Bi2Te3 pseudo-binary system has been reinvestigated experimentally by means of powder X-ray diffraction (XRD), differential thermal analysis (DTA), and scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDX) techniques to obtain a better description of the phase diagram. The new version of the phase diagram differs from previous versions. The formation of a total of 5 tetradymite-type ternary compounds has been experimentally confirmed, including 4 compounds mentioned in the literature and, for the first time, a new compound Sn3Bi2Te6. The newly revealed phase was determined to be a rhombohedral structure (space group R-3m, No. 164, lattice parameters: a = b = 4.40935 angstrom, c = 63.8370 angstrom in hexagonal representation) based on the powder XRD data and melts with solid-phase reaction, while other reconfirmed ternary compounds have peritectic melting character. Furthermore, the solid solubility of starting binary compounds SnTe and Bi2Te3 is established to be -12 and 9 mol % at 450 degrees C, respectively. A eutectic temperature of 575 degrees C was observed at a composition of -85 mol% Bi2Te3. The current updated version of the phase diagram can provide the essential information for alloying strategies and guide the designing of the Sn-Bi-Te-based high-performance functional materials.

Raman spectra of magnetic topological crystalline insulators in a wide temperature range including the magnetic ordering region are studied in detail. The anharmonicity parameters and Grüneisen mode parameters of Raman-active phonons in the studied crystals have been determined. It has been shown that the temperature dependence of the frequency of the A_1g^(1) ( 48 cm –1 ) phonon in MnBi 2 Te 4 coincides within ±0.1 cm –1 with the standard anharmonic model disregarding the spin–phonon coupling. The polarization dependences of Raman spectra in the MnSb 2 Te 4 crystals indicate that Sb and Mn atoms are strongly mixed in them unlike the isostructural MnBi 2 Te 4 crystals.

Systematic studies of magneto-transport properties of the whole (MnBi 2 Te 4 )(Bi 2 Te 3 ) m family of magnetic topological insulators ( m = 0,1,...,6) have been carried out. Temperature dependences of the resistivity, magnetoresistance and the Hall effect at low temperatures have been studied. When m increases, i.e., when the separation between 2D MnBi 2 Te 4 magnetic layers becomes larger, the transition from antiferromagnetic to ferromagnetic state takes place. We have found that ferromagnetic state survives even in the samples with m = 6 , when 2D magnets are separated by six non-magnetic Bi 2 Te 3 blocks.

X-ray studies of TlFeS2 and TlFeSe2 crystals grown by high-temperature synthesis reveal their single-phase nature and isostructurality with the space group C2/m. Raman scattering and infrared reflection of light in TlFeS2 and TlFeSe2 compounds is studied at a temperature of 300 K. Characteristic frequencies in the Raman scattering and infrared spectra of phonons in TlFeS2 and TlFeSe2 are detected. An analysis of published works on the magnetic properties of TlFeS2 and TlFeSe2 allows the conclusion that they are quasi-one-dimensional antiferromagnets each having two characteristic temperatures TN3D and $$T_{c}^{{{\text{super-p}}}}$$ , between which quasi-one-dimensional antiferromagnetic ordering in TlFeS2 and TlFeSe2 is favorable. The temperature $$T_{c}^{{{\text{super-p}}}}$$ is introduced for the first time and characterizes a highly developed short-range magnetic order, in which superparamagnetic ordering exists.

This work is devoted to the experimental study and symmetry analysis of the Raman-active vibration modes in MnBi 2 Te 4 · n (Bi 2 Te 3 ) van der Waals topological insulators, where n is the number of Te–Bi–Te–Bi–Te quintuple layers between two neighboring Te–Bi–Te–Mn–Te–Bi–Te septuple layers. Confocal Raman spectroscopy is applied to study Raman spectra of crystal structures with n = 0,1,2,3,4,5,6 , and ∞. The experimental frequencies of vibration modes of the same symmetry in the structures with different n are compared. The lattice dynamics of free-standing one, three, and four quintuple layers, as well as of bulk Bi 2 Те 3 (n = ∞ ) and MnBi 2 Te 4 (n = 0) , is considered theoretically. Vibrational modes of the last two systems have the same symmetry, but different displacement fields. These fields in the case of a Raman-active mode do not contain displacements of manganese atoms for any finite n . It is shown that two vibrational modes in the low-frequency region of the spectrum (35–70 cm –1 ) of structures with n = 1, 2, 3, 4, 5 , and 6 practically correspond to the lattice dynamics of n free quintuple Bi 2 Те 3 layers. For this reason, the remaining two vibration modes, which are observed in the high-frequency region of the spectrum (100–140 cm –1 ) and are experimentally indistinguishable in the sense of belonging to quintuple or septuple layer or to both layers simultaneously, should also be assigned to vibrations in quintuple layers under immobile atoms of septuple layers.

X ray studies of T1FeS(2) and T1FeSe(2) crystals grown by high-temperature synthesis reveal their single-phase nature and isostructurality with the space group C2/m. Raman scattering and infrared reflection of light in T1FeS(2) and T1FeSe(2) compounds is studied at a temperature of 300 K. Characteristic frequencies in the Raman scattering and infrared spectra of phonons in T1FeS(2) and T1FeSe(2) are detected. An analysis of published works on the magnetic properties of T1FeS(2) and T1FeSe(2) allows the conclusion that they are quasi- one-dimensional antiferromagnets each having two characteristic temperatures T-N3D and T-c(super-P), between which quasi-one-dimensional antiferromagnetic ordering in T1FeS(2) and T1FeSe(2) is favorable. The tempera- ture T-c(super-P) is introduced for the first time and characterizes a highly developed short-range magnetic order, in which superparamagnetic ordering exists.

We study the surface crystalline and electronic structures of the antiferromagnetic topological insulator MnBi 2 Te 4 using scanning tunneling microscopy/spectroscopy (STM/S), micro( μ )-laser angle-resolved photoemission spectroscopy (ARPES), and density functional theory calculations. Our STM images reveal native point defects at the surface that we identify as Bi Te antisites and Mn Bi substitutions. Bulk X-ray diffraction further evidences the presence of the Mn-Bi intermixing. Overall, our characterizations suggest that the defects concentration is nonuniform within crystals and differs from sample to sample. Consistently, the ARPES and STS experiments reveal that the Dirac point gap of the topological surface state is different for different samples and sample cleavages, respectively. Our calculations show that the antiparallel alignment of the Mn Bi moments with respect to those of the Mn layer can indeed cause a strong reduction of the Dirac point gap size. The present study provides important insights into a highly debated issue of the MnBi 2 Te 4 Dirac point gap.

This work is devoted to an experimental investigation of the electronic structure of the surface of topological insulators of various stoichiometry during the adsorption of Co atoms. Changes in the surface electronic structure of Bi2Te3 and MnBi2Te4 systems upon deposition of Co atoms at various temperatures have been studied using the methods of angle-resolved photoemission spectroscopy, as well as X-ray photoelectron spectroscopy. It is suggested that binding of the adsorbed Co atoms to the substrate surface modifies Dirac point position. The observed changes are associated with the possible formation of magnetic Co-containing ordered surface alloys.

Further to the structure of the intrinsic magnetic topological insulators $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}\ifmmode\cdot\else\textperiodcentered\fi{}n({\mathrm{Bi}}_{2}{\mathrm{Te}}_{3})$ with $n<4$, where index $n$ is the number of quintuple Te-Bi-Te-Bi-Te building blocks inserted between the neighboring septuple Te-Bi-Te-Mn-Te-Bi-Te building blocks, the structure of the members with $n=4$, 5, and 6 was studied using x-ray powder diffraction. The unit-cell parameters and atomic positions were obtained. The obtained and available structural data were summarized to show that the crystal structure of all members of $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}\ifmmode\cdot\else\textperiodcentered\fi{}n({\mathrm{Bi}}_{2}{\mathrm{Te}}_{3})$ follows the cubic close-packing principle, independently of the space group of the given member. Confocal Raman spectroscopy was then applied. Comparative analysis of the number, frequency, symmetry, and broadening of the vibration modes responsible for the lines in the Raman spectra of the systems with $n=1$,\dots{},6, as well as $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}\phantom{\rule{0.16em}{0ex}}(n=0)$ and ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ ($n=\ensuremath{\infty}$) has shown that lattice dynamics of $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}\ifmmode\cdot\else\textperiodcentered\fi{}n({\mathrm{Bi}}_{2}{\mathrm{Te}}_{3})$ with $n>0$ overwhelmingly dominates by the cooperative atomic displacements in the quintuple building blocks.

This work is devoted to the experimental study and symmetry analysis of the Raman-active vibration modes in MnBi2Te4·n(Bi2Te3) van der Waals topological insulators, where n is the number of Te–Bi–Te–Bi–Te quintuple layers between two neighboring Te–Bi–Te–Mn–Te–Bi–Te septuple layers. Confocal Raman spectroscopy is applied to study Raman spectra of crystal structures with $$n = 0,1,2,3,4,5,6$$ , and ∞. The experimental frequencies of vibration modes of the same symmetry in the structures with different n are compared. The lattice dynamics of free-standing one, three, and four quintuple layers, as well as of bulk Bi2Те3 $$(n = \infty )$$ and MnBi2Te4 $$(n = 0)$$ , is considered theoretically. Vibrational modes of the last two systems have the same symmetry, but different displacement fields. These fields in the case of a Raman-active mode do not contain displacements of manganese atoms for any finite $$n$$ . It is shown that two vibrational modes in the low-frequency region of the spectrum (35–70 cm–1) of structures with $$n = 1,\;2,\;3,\;4,\;5$$ , and 6 practically correspond to the lattice dynamics of n free quintuple Bi2Те3 layers. For this reason, the remaining two vibration modes, which are observed in the high-frequency region of the spectrum (100–140 cm–1) and are experimentally indistinguishable in the sense of belonging to quintuple or septuple layer or to both layers simultaneously, should also be assigned to vibrations in quintuple layers under immobile atoms of septuple layers.