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Scientific Reports Volume 6, Article Number: 19154 (2016) Cite this article
A transmission electron microscope experiment was used to characterize the structural fluctuations of the hexagonal Ba1-xSrxAl2O4 with a corner-sharing AlO4 tetrahedral network at different temperatures. For x ≤ 0.05, the soft mode and equivalent wave vector of q ~ (1/2, 1/2, 0) condense at the transition temperature (TC), forming a superstructure with a unit volume of 2a × 2b × c. However, TC is largely inhibited by Sr substitution and disappears when x ≥ 0.1. In addition, the q ~ (1/2, 1/2, 0) soft mode deviates from the corresponding value as the temperature decreases, and survives in the nano-scale region below ~200 K. These results strongly indicate the existence of soft patterns. Two different soft modes as honeycomb diffuse scattering are observed in high temperature regions up to 800 K. This inherent structural instability is a unique feature of framework compounds and is the cause of this abnormal fluctuating state.
The new phase near the ordered state of electron spin, charge, or orbit has long been a fascinating topic in condensed matter physics. Notable examples are superconductivity, charge density wave (CDW), spin density wave (SDW), and orbital order that are close to antiferromagnetic order, which are usually found in cuprates, transition metal dichalcogenides 1. In iron arsenide 2 and ruthenate 3. The knowledge accumulated from these studies provides us with a simple and universal description, that is, abnormal states often appear from the fluctuations in the order state of spin, charge, or orbit. Just like electrons, phonons are the fundamental quantum in crystals. Although a large number of experimental and theoretical studies have focused on the fluctuations of the three aspects of electrons, there are few studies on the new phenomenon that "phonon fluctuations" play a key role.
In skeletal compounds containing linked polyhedrons, for example, several variants of silicate 4,5, nepheline 6, and ZrW2O87, the existence of polyhedral-related motion within the network structure has been reported. These related movements are called "rigid element modes" (RUM) 8,9,10. These RUMs sometimes act as soft modes, inducing structural phase changes, such as those in quartz 4, tridymite 5, and nepheline 6. For the framework compound BaAl2O4, some people think that RUM is the main structural instability11.
BaAl2O4 crystallizes into a squamous crystal structure 12, 13, 14, 15, which includes a three-dimensional network of AlO4 tetrahedrons sharing angles, and the six-element cavities of the tetrahedral network are occupied by alkaline earth metal ions. In the electron diffraction pattern of its high temperature phase, characteristic honeycomb diffuse scattering (honeycomb pattern) has been observed. Since the scattering intensity is strongly dependent on temperature, the characteristic honeycomb pattern may originate from the inherent structural fluctuations associated with soft modes. Recent studies on structural phase transitions using synchrotron X-ray diffraction have shown that the system has two types of soft modes, both of which generate strong diffuse scattering intensities, which increase sharply toward the structural phase transition temperature (TC) , Indicating that the two modes are condensed at the same time in TC17. In addition, the compound exhibits inappropriate ferroelectricity, accompanied by a structural phase transition of approximately 400 K12,13,18. According to reports, this transition temperature is rapidly reduced by disorder of AE sites, for example, Sr partially replaces Ba19. Inspired by the analogy of spin, charge, or orbital order state fluctuations, we studied the structure fluctuations of the disordered BaAl2O4 of the AE position.
In this study, the diffuse scattering temperature change and structural phase transition temperature of Ba1-xSrxAl2O4 were systematically studied by transmission electron microscopy (TEM). We report the unusual state of Ba1−xSrxAl2O4 below 200 K, where x ≥ 0.1, where dynamic structural fluctuations develop as the temperature decreases.
The high temperature phase of AEAl2O4 (AE = Sr or Ba) crystallizes into a hexagonal structure with space group P6322, as shown in Figure 1a. The low-temperature ferroelectric phase of BaAl2O4 is determined to be a hexagonal P63 superstructure under TC, ah = 2ap, bh = 2bp and ch = cp, where the subscripts h and p represent the hexagonal low-temperature structure and high-temperature matrix structure, respectively. SrAl2O4 crystallizes into P21 monoclinic structure below 950 K15. The unit settings of P6322, P63 and P21 are shown in Figure 1b. The subscript m represents the P21 monoclinic structure. Figure 1c shows the powder X-ray diffraction (XRD) curve of Ba1-xSrxAl2O4 (x = 0 and 0.05) at room temperature in the range of 2θ = 35-38°. The superlattice reflection of the P63 low-temperature phase marked with an arrow can be clearly observed at x = 0, but cannot be observed at x = 0.05. This indicates that the sample with x = 0 crystallizes with the P63 crystal structure at room temperature, while the sample with x = 0.05 has the symmetry of P6322. The high-resolution TEM (HRTEM) experiment of the sample with x = 0 also shows that the fine phase domain with P63 symmetry covers the entire area of the crystal, as shown in the online supplementary Figure S1. The samples with 0.05 ≤ x ≤ 0.62 crystallize in the P6322 matrix structure at room temperature. Figure 1d shows the relationship between the lattice parameters at room temperature and the nominal Sr content x. With the increase of x, the linear decrease of these parameters indicates the systematic substitution of Sr, whose ionic radius is smaller than Ba. The P21 monoclinic structure appears above x = 0.64. Therefore, we focus on the structural phase transition of samples with x <0.6 from P6322 to P63.
The structure change of Ba1−xSrxAl2O4.
(a) The crystal structure of the hexagonal P6322 matrix of BaAl2O4. (b) Various battery settings for BaAl2O4 and SrAl2O4. The thick solid line and the gray line represent the unit cell of the P6322 matrix structure and the P63 low-temperature phase of BaAl2O4, respectively. The dotted line indicates the P21 low temperature phase of SrAl2O4, which has the same crystal structure as BaAl2O4 at high temperatures. (c) Powder XRD spectrum of Ba1−xSrxAl2O4 (x = 0 and 0.05) polycrystalline sample at room temperature in the range of 2θ = 35-38°. The superlattice reflection is observed in the sample with x = 0, as indicated by the arrow. (d) The change in lattice parameters at room temperature plotted relative to the nominal Sr substitution level x. The circle, triangle, and diamond symbols represent the lattice parameters of the crystal structures of P63, P6322, and P21, respectively. (e) 1/2 3/2 1 The temperature dependence of the reflection intensity of the superlattice, normalized by the intensity of the 111 elementary reflection. The solid line is the fitted curve to show the trend. The error bar is the standard deviation. The arrow indicates the TC value. (f) FWHM as a function of temperature. (g) The powder XRD spectrum near the 1/2 3/2 1 superlattice reflection of Ba1-xSrxAl2O4 obtained at 100 K, where x = 0-0.06.
Figures 1e and f show the 1/2 3/2 1 superlattice strength and the full width at half maximum (FWHM) of the x = 0, 0.03, and 0.05 samples, respectively, as a function of temperature. The data is obtained during the heating process. The superlattice intensity is normalized using the intensity of 111 elementary reflections (Int111). For x = 0, the structural phase transition occurs at 420 K, the superlattice strength suddenly decreases, and at the same time the FWHM increases. The transition temperature decreases as x increases, which is consistent with the previous report. Figure 1g shows the 1/2 3/2 1 superlattice strength at 100 K for x = 0–0.06. The strength of the superlattice decreases as x increases, while the FWHM increases, as shown in Figure 1g. This result shows that the correlation length of the superstructure decreases as the Sr concentration x increases. For x = 0.06, the weak intensity of 1/2 3/2 1 results in broad peaks, even though they are largely independent of temperature. For each component, the discontinuity of lattice parameters cannot be detected at TC.
In order to study the structural fluctuation of Ba1-xSrxAl2O4, electron diffraction experiments were carried out between 100 and 800 K during the heating process. Figure 2a shows the [1-10] Axial Electron Diffraction pattern obtained at 293 K (left image) and x = 0 at 100 K (right image). The superlattice reflection at the reciprocal position of h 1/2 k 1/2 l, marked by the arrow in Figure 2a, represents the P63 superstructure. However, the superlattice reflections at x = 0.05 are wider than those at x = 0; these reflections at x = 0.05 are slightly elongated in the [110] direction. x = 0 and 1/2 1/2 of 0.05 5-The superlattice reflection scans along [110] in the illustration. The FWHM of the superlattice reflection at x = 0.05 is significantly greater than the FWHM at x = 0. Surprisingly, diffuse scattering occurs around the superlattice reflection of x = 0 at 310 K, as shown in the online supplementary figure S2, which is 110 K lower than TC. Above TC, two different diffuse scattering are observed near the reciprocal positions of h 1/2 k 1/2 l and h 1/3 k 1/3 l, and the diffuse fringes between them (see Supplementary Fig. S2 And S3 online). Recent synchrotron X-ray diffraction experiments and first-principles calculations show that these diffuse scattering are due to q ~ (1/2, 1/2, 0) and q'~ (1/3, 1/3, 0) soft Mode 17. That is to say, due to structural instability, two different soft modes coexist on TC. To our knowledge, so far, no such coexistence has been reported in other compounds.
The diffuse scattering of Ba1−xSrxAl2O4.
(a) The diffraction pattern of x = 0 single crystal at 293 K (left) and x = 0.05 polycrystalline sample at 100 K (right) at [1-10] incidence. Due to multiple reflections, a forbidden reflection of 00l (l = 2n 1) is observed. The inset shows the line scan along 1/2 1/2 5-superlattice reflection of [110]. (b) [1-10] The temperature change of the incident diffraction pattern when x = 0.1. The arrows indicate diffuse scattering. (c) A schematic diagram of the relationship between the reciprocal vector Kh and the wave vectors q ~ (1/2, 1/2, 0) and q'~ (1/3, 1/3, 0).
In contrast, Ba1−xSrxAl2O4 (0.1 ≤ x ≤ 0.5) exhibits an unusual state below ~200 K, in which soft modes fluctuate in k-space. In this state, the low-energy soft mode has three symmetrically equivalent qs, q1 ~ (1/2, 1/2, 0), q2 ~ (1, -1/2, 0) and q3 ~ (- 1/ 2, 1, 0) In the reciprocal space, it deviates from the corresponding q as the temperature decreases. Figure 2b and Supplementary Figure S4 show online the temperature change of the electron diffraction pattern with x = 0.1 obtained from the 100 K heating period. Over 200 K, diffuse scattering near h 1/2 k 1/2 l and the reciprocal position of h 1/3 k 1/3 l appear. At 190 K, the diffuse scattering near the reciprocal position of h 1/2 k 1/2 l still exists, as shown by the arrow in the figure, while the diffuse scattering near h 1/3 k 1/3 l disappears from each other. In addition, these diffuse scattering will change shape as the temperature decreases; they elongate significantly at 100 K. This strongly indicates that the soft mode q ~ (1/2, 1/2, 0) will not condense to form the P63 superstructure, but will exist in a fluctuating state. These changes are shown schematically in Figure 2c. The significant extension of the diffuse scattering edge [110] means the deviation of the q ~ (1/2, 1/2, 0) wave vector from the corresponding wave vector. It has been confirmed that these changes occur reversibly during thermal cycling. For x = 0.3, 0.4, and 0.5, similar results were obtained.
The structural changes of Ba1-xSrxAl2O4 are summarized in Figure 3a (Figure 3b-e) together with the [001] Axial Electron Diffraction pattern. The solid black line represents the TC determined by XRD. The dashed line with x ≤ 0.05 corresponds to Ts, below which the diffuse scattering disappears. For the temperature between TC and Ts, in addition to the superlattice reflection, diffuse scattering near h 1/2 k 1/2 l and h 1/3 k 1/3 l is also observed, as shown in Figure 3b ( See also online supplementary figures S2 and S3). Tf represents the temperature at which the q vector of the soft mode deviates from the corresponding value. For all compositions, the coexistence of two diffuse scattering is observed in a wide temperature region above TC and Tf, which produces characteristic honeycomb patterns. For x = 0.5, this coexistence is observed even at 798 K, as shown in the online supplementary Figure S5. The area above TC and Tf is denoted as "diffuse reflection 1" in Figure 3a, and the area below Tf is denoted as "diffuse reflection 2".
(a) The structural phase diagram of Ba1-xSrxAl2O4. The solid triangle corresponds to the TC determined from the XRD experiment. Ts (open circle) when x ≤ 0.05 and Tf (open diamond) when x ≥ 0.1 means that the diffuse scattering near the reciprocal position of h 1/3 k 1/3 l disappears when the temperature is lower than that. (be) Typical electron diffraction pattern of each area. The superlattice reflection is marked with a yellow circle in the electron diffraction pattern. In the area above TC and Tf (denoted as "diffuse reflection 1"), the diffuse scattering near h 1/2 k 1/2 l (red ellipsoid) and h 1/3 k 1/3 l (green ellipsoid) ) Observe the mutual position and the diffuse fringes between them. In the area below Tf, denoted as "diffuse reflection 2", only diffuse scattering near h 1/2 k 1/2 l is observed. (f,g) Dark-field TEM image, including corresponding electron diffraction patterns at 293 K and 104 K, respectively, for x = 0.3, using a circle of h 1/2 k 1/2 l diffuse scattering represented by the dashed line.
In order to understand the structural changes of Tf in depth, use TEM to observe the microstructure on Ba1-xSrxAl2O4 with x = 0.3. The high-resolution TEM image of 293 K shows uniform lattice fringes without additional feature contrast, such as the structure inverted boundary (see online supplementary figure S6). In addition, the fast Fourier transform calculation of the high-resolution TEM image reproduces the experimentally obtained electron diffraction pattern, which includes two different diffuse scattering near h 1/2 k 1/2 l and h 1/3 k 1/3. In addition to basic reflection, the mutual position and diffusion fringes between them. Figures 3f and g show the dark-field TEM images and corresponding electron diffraction patterns of samples with x = 0.3 obtained at 293 and 104 K, respectively. One of the h 1/2 k 1/2 l diffuse scattering is used as indicated in Figure 3f The dotted circle indicates that g is in two beam conditions. There is a uniform black area covering the entire area of 293 K, as shown in Figure 3f, indicating that there is no superstructure in the diffuse reflection 1 area. The alternating darker and lighter fringes observed in the wedge-shaped area are due to the thickness fringes. However, the island-like nanoscale area with a width of about 20 nm was observed as black or white spots at 104 K, as shown in Figure 3g. Since the diffuse scattering near the reciprocal position of h 1/2 k 1/2 l strongly depends on the temperature below ~200 K, it is considered that q ~ (1/2, 1/2, 0) soft is a reasonable mode at each It still exists in the nano-scale area.
Since q ~ (1/2, 1/2, 0) soft mode does not condense, but fluctuates as the temperature decreases, it is expected that no structural phase change will occur at lower temperatures. Such nanoscale regions 20, 21, 22, 23, 24 have been observed in several compounds. For example, in AlV2-xCrxO420,21, the charge order state is suppressed by Cr doping, and a spin-glass ground state appears. Among TiSe223 and CuxTiSe224, some people believe that the onset of superconductivity is related to the formation of domain walls in the CDW sequence25. Our results strongly indicate the existence of quantum criticality, that is, the structural fluctuation state remains intact until absolute zero temperature without phase change. Recently, the structural quantum criticality of the iron arsenide superconductor Ba(Fe1-xCox)2As2 has been discussed, in which the elastic constant softens in the vicinity of the superconductor. Although Ba1−xSrxAl2O4 is a good insulator, a small amount of non-stoichiometry of Ba and O has been reported in this system. This precise non-stoichiometric control is believed to open the way for the development of new functional materials with strong coupling of conduction electrons and structural fluctuations.
The large structural fluctuations of this compound can be attributed to the inherent structural instability caused by RUMs8,9,10 in the framework compound with corner-sharing polyhedrons. These modes are well known in the modification of SiO2, for example, α-quartz and β-tridymite, where RUM acts as a soft mode and causes disproportionate or continuous phase changes 4,5. In BaGa2O4 filled tridymite-type compounds, the inherent structural instability is related to the mismatch between the average distance and the distance between atoms. In Ba1−xSrxAl2O4, because atoms occupy the hexagonal channels in the tridymite crystal structure, slight changes in structural instability will lead to the coexistence of two different soft modes in the high temperature region and the fluctuating low temperature region dominated by nanoscale regions. In order to clarify the nature of these fluctuating states and the mechanism of Ba1-xSrxAl2O4 fluctuations, the use of synchrotron XRD experiments until the structural improvement of the liquid He temperature is in progress.
In summary, we found that the ordered phase with P63 superstructure in BaAl2O4 is rapidly inhibited by the substitution of Sr for Ba. q ~ (1/2, 1/2, 0) The abnormal fluctuation state of the soft mode increases to less than 200 K within the composition range between x = 0.1 and 0.5. The nanoscale region dominates this composition window, indicating that no structural phase transition has occurred to the ground state. The inherent structural instability in the AlO4 tetrahedral network is the cause of this low-temperature fluctuation state.
Polycrystalline samples of Ba1−xSrxAl2O4 (0 ≤ x ≤ 1) were synthesized using traditional solid-state reactions. BaCO3 (99.9%), SrCO3 (99.9%) and Al2O3 (99.99%) powders are mixed at a molar ratio of 1:1 and calcined at 1200 °C for 10 hours and in air at 1300 °C for 12 hours. Grinding. The resulting powder is pressed into granules, sintered at 1450°C for 48 hours, and then furnace cooled to room temperature. These particles are stored in a vacuum. The self-fusion method was used to prepare BaAl2O4 single crystals in a platinum crucible. The previously synthesized mixture of BaAl2O4 and BaCO3 with a molar ratio of 50:17 was heated at 1470°C for 6 hours, and then slowly cooled to 1200°C at a rate of 2°C/h. After the furnace is cooled to room temperature, colorless and transparent hexagonal crystals with an edge length of about 100 μm are mechanically separated from the flux. The samples obtained from the flux are stored in a vacuum. Use Cu-Kα radiation for powder XRD in the temperature range between 100 and 500 K. The powder XRD distribution shows that all polycrystalline samples are single-phase samples. Single crystal XRD experiments also show that the crystal has a P63 crystal structure at room temperature. Using the dual-tilted liquid N2 cooling bracket and heating bracket in the JEM-2010 TEM system (JEOL, Japan), in-situ TEM observations were performed in the temperature range of 100 to 800 K. After reaching the desired temperature, collect all diffractograms at intervals of more than 30 minutes until no change in the diffractogram is observed. All indices and electron diffraction patterns in the XRD spectrum are based on the P6322 parent phase in this paper.
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This work was partially supported by a scientific research grant from the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT).
Department of Materials Science, Osaka Prefecture University, Sakai, 599-8531, Osaka, Japan
Yui Ishii, Hirofumi Tsukazaki, Eri Tanaka and Shigeo Mori
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YI and SM conceived and designed this study. YI and ET synthesized these materials. HT and SM conducted TEM experiments. YI analyzed all the data and wrote the paper. YI and SM contributed to the discussion of the results.
The author declares that there are no competing economic interests.
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Ishii, Y., Tsukasaki, H., Tanaka, E. etc. The fluctuating state in the framework compound (Ba,Sr)Al2O4. Scientific Report 6, 19154 (2016). https://doi.org/10.1038/srep19154
DOI: https://doi.org/10.1038/srep19154
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