熔盐法生长Nd:KGW多波长激光晶体结构及光谱性能的研究

采用熔盐法,以K2W2O7为助溶剂,优化了晶体生长的工艺参数,生长出了新型稀土激光晶体Nd∶KGW。采用XRD及X射线荧光分析,确认了所得到的晶体为β Nd∶KGW晶体。通过TG DTA分析和测量,得到了其熔点及相变温度分别为1086℃和1021℃。利用红外光谱和Raman光谱确定了其分子基团的振动归属。通过测量其吸收光谱,该晶体在波长为808nm处有强吸收峰,可以与激光二极管有效地耦合。通过计算,获得了其峰值吸收截面积。测试了该晶体的荧光光谱,所得到的晶体发射波长为1.06μm和1.35μm。     

Pb-Sn合金及Pb的热力学性质的第一性原理计算

本文基于第一性原理的密度泛函理论(DFT)和密度泛函微扰理论(DFPT)方法,利用虚晶近似的计算方法研究了Pb-Sn合金的晶格结构、电子能带、声子能带及热力学性质,并用晶格能量差可与达到熔化温度时的振动能量相当的固-液相变机理研究了熔化温度,同时与所计算Pb的所有结果进行了对比.     

硫酸锂钾的室温相和低温相的锂离子位置

核磁共振实验表明,LiKSO₄在T_c=195K发生结构相变。本文报道了温度低于T_c时,Li位电场梯度张量的温度依赖关系。结合理想离子晶体模型,讨论了该晶体的结构,指出室温相Li的位置并不像Bradley所推测的,在(2/3,1/3,0.849)的位置,而应在(2/3,1/3,0.706)的位置;195K的相变是由于Li离子的位移造成;低温相的空间群可能为R3。 关键词：     

Phase transition in a NaBi(MoO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystal: Acoustic investigations

This paper reports on the results of acoustic investigations of a NaBi(MoO₄)₂ crystal in the temperature range from 20 to 70°C. The temperature dependences of the velocity of longitudinal ultrasonic waves propagating along the crystallographic axes z and x are measured at a frequency of 4 MHz. The results obtained demonstrate that a structural phase transition occurs in the NaBi(MoO₄)₂ crystal at a temperature of 309 K. The experimental findings are consistent with the assumption that the observed phase transition is either a second-order ferroelastic transition or a first-order ferroelastic transition that is very close to being a second-order phase transition.     

Na2CO3的高压拉曼光谱研究

碳酸盐是碳在地球内部的重要载体之一,其在地幔高温高压条件下的晶体化学是理解地球深部碳的赋存状态和循环过程的关键,而结构稳定性和相变是晶体化学最基本的研究内容。碳酸钠（Na₂CO₃）是一种常见的碱性碳酸盐矿物,在产自地幔过渡带-下地幔的金刚石中已发现含钠的碳酸盐矿物包裹体,这成为碳酸钠能够俯冲进入地幔深部的直接矿物学证据。前人利用拉曼光谱技术研究了Na₂CO₃在常温常压下的晶格振动模式,但其在高压下的稳定性和结构变化却鲜有报道。利用金刚石压腔装置结合先进的共聚焦拉曼光谱技术,以硅油作为传压介质,在准静水压力条件下,在0.001～27.53 GPa压力区间对Na₂CO₃粉末在600~1 200 cm-1波段的振动特征进行了细致地分析。本次实验重点分析了[CO₃]2-基团振动模式在升压和卸压过程中的行为。结果表明,在0.001～11.88 GPa压力范围内,[CO₃]2-基团对称伸缩振动γ₁（1 088.06和1 070.76 cm-1）、反对称伸缩振动γ₃（865.10和797.50 cm-1）和面内弯曲振动γ₄（720.10和696.71 cm-1）都出现了振动峰的分裂。随着压力增加,所有振动峰都向高频率漂移,半高宽也逐渐增加。在13.40 GPa时,Na₂CO₃发生结构相变,具体表现为690.08 cm-1处出现1条新的拉曼峰,并且随着压力升高该峰的强度逐渐增大。同时反对称伸缩振动峰γ₃以及面内弯曲振动峰γ₄的强度持续减弱,半高宽也继续变大。这些现象表明Na₂CO₃结构相变源于[CO₃]2-内部晶格变化。当压力卸载到4.18 GPa时,[CO₃]2-的振动模式与常温常压下的完全吻合,相变出现的新峰也已经消失,表明该相变是由[CO₃]2-基团畸变引起的并且具有可逆性。继续升压至27.53 GPa,拉曼光谱继续蓝移,Na₂CO₃的拉曼谱线再没有变化,说明高压相在这一压强范围内保持稳定。在整个加压过程中,反对称伸缩振动γ₃和面内弯曲振动γ₄处的拉曼峰出现强度减弱现象。同时也计算了各个峰频率对压力的依赖系数dγ/dP,结果显示[CO₃]2-基团内各个振动模式对压力的响应是不同的,这很可能与Ｃ-Ｏ键的键长有关。最后,对比发现,对称伸缩振动γ₁峰的强度比反对称伸缩振动γ₃和面内弯曲振动γ₄峰的强度大,并且[CO₃]2-基团对称伸缩振动γ₁受压力影响相对较小,可以用来区别不同种类的碳酸盐矿物。     

SO2-4/TiO2固体酸的红外和拉曼光谱研究

用ＩＲ、Ｒａｍａｎ光谱研究了ＳＯ＾２－４／ＴｉＯ２固体酸在不同烧温度下的结构、晶相转变和表面酸中心。结果表明,ＳＯ＾２－４与ＴｉＯ２表面的结合为螯合式双配结构。当烧结温度小于５００℃时,ＳＯ＾２－４／ＴｉＯ２样品具有较高的结构稳定性,晶相结构以锐钛矿为主,表面Ｂ酸位数目约是Ｌ酸位数目的２倍,当烧结温度大于５００℃时,随着烧结晶度的升高,表面结合的ＳＯ＾２－４逐渐流失,晶相从锐钛矿转变为金红石,表面Ｂ酸位减少并消失。     

Phase transition and ferroelastic property studied by using the Cs nuclear magnetic resonance in a CsHSO4 single crystal

The ¹³³Cs spin-lattice relaxation time in a CsHSO₄ single crystal was measured in the temperature range from 300 to 450 K. The changes in the ¹³³Cs spin-lattice relaxation rate near T_c1 (=333 K) and T_c2 (=415 K) correspond to phase transitions in the crystal. The small change in the spin-lattice relaxation time across the phase transition from II to III is due to the fact that during the phase transition, the crystal lattice does not change very much; thus, this transition is a second-order phase transition. The abrupt change of T₁ around T_c2 (II-I phase transition) is due to a structural phase transition from the monoclinic to the tetragonal phase; this transition is a first-order transition. The temperature dependences of the relaxation rates in phases I, II, and III are indicative of a single-phonon process and can be represented by T₁⁻¹=A+BT. In addition, from the stress-strain hysteresis loop and the ¹³³Cs nuclear magnetic resonance, we know that the CsHSO₄ crystal has ferroelastic characteristics in phases II and III.     

Reflection of elastic waves in a crystal of Heusler alloy Ni<Subscript>2</Subscript>MnGa in the phase transition range

The reflection of longitudinal and transverse acoustic waves from the free surface of the ferromagnetic shape memory alloy Ni₂MnGa that is located in the ranges of the premartensite and martensite phase transformations is considered. The propagation directions and amplitudes of the waves reflected in the (001) plane of the crystal are determined. They acquire the character of substantially quasi-longitudinal and quasi-transverse vibrations rather than being pure modes. The angles of wave reflection and conversion are shown to be effectively controlled by temperature and a magnetic field due to the colossal acoustic anisotropy of the crystal over the wide range of its phase transitions. Beginning from a certain critical angle of incidence of a quasi-transverse wave, the quasi-longitudinal wave having appeared upon reflection becomes an accompanying surface vibration, and it can be emitted into the bulk of the crystal when the phase transition point is approached. Two angles of full conversion of an incident quasi-longitudinal wave into a quasi-transverse wave are established, and their temperature dependences are found. Trivisonno’s experimental data for the ultrasound velocity and absorption in an Ni₂MnGa crystal are used to numerically estimate these acoustic effects.     

RAMAN STUDIES ON THE PHASE TRANSITIONS OF (NH4)2SnBr6 CRYSTALS

We report here the structural phase transitions of the crystal (NH₄)₂SnBr₆ investigated by Raman scattering at temperatures ranging from 10K to room temperature. Two phase transitions occurring at 150 and ll0K are found. Based on the group theory, it is proposed that the crystal undergoes a second-order phase transition at 150 K, resulting from a ferro-distortion with symmetry Γ⁴⁺. The change of structure is confirmed to be O⁵_h to C⁵_4h, which is assigned to the rotary of [SnBr₆]^2- ion groups around the axis of <001>. Furthermore the crystal undergoes an order-disorder phase transition at ll0K which is related with the reorientation of the ammonium ion group. It is noticed that the change of the vibrational modes at 77K does not show any phase transition.     

Acoustic emission at phase transition in vanadium sesquioxide single crystals

The metal-semiconductor phase transition in vanadium sesquioxide is investigated by the acoustic emission method. It is shown that the acoustic emission in single crystals of this compound is due to thermoelastic stresses arising in the crystal upon the phase transition. Transformation of the acoustic emission activity and an increase in the phase transition temperature are revealed in the temperature cycling of the sample. Observation of peaks of the acoustic emission activity at a temperature of 5–6 K above the critical temperature indicates that crystal nuclei of the monoclinic phase appear in the high-temperature (trigonal) phase of the crystal.     

A neutron diffraction study of ferroelectric PbHAsO4 at room temperature

A single-crystal neutron-diffraction structural study of PbHAsO₄ in its ferroelectric phase at room temperature (24°C) is presented as part of an investigation of a family of monetites. The crystal structure of PbHAsO₄, and its phase transition mechanism are discussed. Comparison of the crystal structures of PbHPO₄ and PbHAsO₄ reveals that these two ferroelectrics are isomorphous. The role of protons in structural phase transition of PbHAsO₄ is also discussed.     

Crystal melting processes of propylene carbonate and 1,3-propanediol investigated by the reed-vibration mechanical spectroscopy for liquids

The melting of crystals is one of the most common and general phase transition phenomena.However, the mechanism of crystal melting is not well understood, and more experimental measurements and explorations are still needed.The mechanical spectra of propylene carbonate and 1,3-propanediol during the crystal melting processes are measured by the reed vibration mechanical spectroscopy for liquids(RMS-L) for the first time.The experimental results show that as the temperature increases, the real part of the complex Young modulus first decreases slowly, and then quickly drops to zero;meanwhile, its imaginary part increases slowly at first, then goes up and drops quickly to zero, showing a peak of internal friction.Preliminary analyses indicate that both the real and imaginary parts can present some characteristics of the melting process, such as the transition from the disconnected liquid regions to the connected liquid regions, that from the connected crystal regions to the disconnected crystal regions, and so on.In addition, the results show that the melting rate per unit volume of crystalline phase versus temperature satisfies the Arrhenius relation at the initial stage of melting, and deviates from this relation as the temperature increases to a certain value.Therefore, the RMS-L will provide an effective supplement for the further study of melting.     

Ca$lt;sub$gt;0.64$lt;/sub$gt;WO$lt;sub$gt;4$lt;/sub$gt;:Eu$lt;sub$gt;0.24$lt;/sub$gt;陶瓷的高温相变机理及其对发光性能的影响

纯的CaWO₄具有优异的耐压、耐热稳定性,化学组成为Ca_0.64WO₄:Eu_0.24的陶瓷也具有CaWO₄结构,但Ca²⁺晶格位置含有12 mol%的肖特基缺陷. 这种缺陷浓度高的CaWO₄ 相是否具有良好的高温稳定性还有待研究. 本文探讨了过度烧结对Ca_0.64WO₄:Eu_0.24陶瓷相结构的影响,揭示了在高温下产生相变的可能原因,并研究了该相变对材料发光性能的影响. 研究表明,当烧结温度超过1100 ℃时,被肖特基缺陷束缚的部分氧离子会解离,造成Ca_0.64WO₄:Eu_0.24陶瓷体相中氧元素含量严重不足,诱发CaWO₄相发生相变,析出单斜晶系的Eu₂WO₆;研究还发现,CaWO₄相的晶面间距在高温相变后会增大;这可能是导致Ca_0.64WO₄:Eu_0.24陶瓷发光强度显著降低的一个重要原因. 关键词： 相变 钨酸钙 铕 发光     

Crystal stability and the theory of ferroelectricity part II. Piezoelectric crystals

In Part I the dielectric properties of cubic crystals with atoms in special positions were discussed, with particular reference to crystals which become unstable against a transverse optic mode of vibration of long wavelength. These considerations are now extended to crystals of general symmetry, and it is shown that the anomalous temperature dependence of certain dielectric, piezoelectric, and elastic constants in the paraelectric phase of ferroelectric crystals can be described and correlated in a very natural way in terms of crystal stability. A formula relating the dispersion frequency of the anomalous dielectric constant to the spontaneous polarization and the displacements of the atoms in a ferroelectric transition, previously derived for barium titanate, is generalized. The transitions in KH₂PO₄ and in NH₄H₂PO₄, and the ordering of the hydrogen atoms in these crystals, are discussed qualitatively in terms of the stability of normal modes of vibration.     

高温高压下方解石相转变的拉曼光谱原位实验研究

用热液金刚石压腔装置结合拉曼光谱技术研究了高温高压下方解石的相变过程及拉曼光谱特征。结果表明:常温条件下,体系压力增至1 666和2 127 MPa时,方解石的拉曼特征峰155cm-1消失,1 087cm-1峰分裂为1 083和1 090cm-1两个谱峰、282cm-1峰突然降至231cm-1,证明其转变为方解石-Ⅱ和方解石-Ⅲ。在起始压力为2 761MPa和低于171℃的升温过程中,方解石-Ⅲ的拉曼散射的各个特征振动峰没有变化。当温度达到171℃,方解石晶体完全变成不透明状,其对称伸缩振动峰1 087cm-1、面内弯曲振动峰713cm-1和晶格振动峰155和282cm-1均发生突变,说明方解石-Ⅲ相变生成一种碳酸钙新相。体系降至常温,该新相一直保持稳定不变,表明高温高压下方解石向碳酸钙新相的转变过程是不可逆的。方解石-Ⅲ与碳酸钙新相之间的相变线方程为P(MPa)=9.09.T(℃)+1 880。碳酸钙新相的对称伸缩振动峰(ν1 087)随压力、温度的变化率分别为dν/dP=5.1(cm-1.GPa-1),dν/dT=-0.055 3(cm-1.℃-1)。     

环丁醇温致相变的原位拉曼光谱研究

采用拉曼光谱和红外光谱解析了常温条件下环丁醇的各个振动模式及其与分子构象间的关联,结果表明液态环丁醇以赤道–反式构象为主,并含有少量的赤道间扭式构象。在此基础上结合差示扫描量热技术和变温拉曼光谱,原位研究了环丁醇的温致相变过程和分子构象随温度的变化。结果表明,冷却至140 K的过程中,环丁醇并未结晶固化,而是保持亚稳定的无序液体状态,即出现过冷现象,继续降温至138 K出现玻璃化转变。升温过程中,在170 K时出现放热峰,同时有新的拉曼峰出现,并且拉曼峰的半高宽和强度发生明显的突变,表明环丁醇由无序结构转变为有序的结晶相。因此,我们获得了环丁醇的温致相变序列： 液态→过冷液体→玻璃态→结晶态→液态。通过对环丁醇不同分子构象的拉曼特征峰的定量分析,证实环丁醇在降温过程中,反式构象和间扭式构象的比例未发生明显改变,即没有发生构象变化。然而升温时,伴随170 K时结晶态的转变,反式构象特征峰的相对强度减小,表明部分分子由赤道–反式构象转变为赤道–间扭式构象。该研究结果对进一步理解和研究其他有机小分子的温致相变和构象变化具有指导意义。     

Shifting of transition temperature of ferroelectric liquid crystals due to addition of dye-An optical and dielectric study

In order to study the shifting of phase transition temperature of ferroelectric liquid crystals due to addition of dye molecules, we have investigated two ferroelectric liquid crystal materials (Felix 16/030 and Felix 16/100) and their five mixtures with Anthraquinone dye (1%, 2%, 3%, 4% and 5% wt/wt). The phase transition scheme has been investigated and analyzed by results obtained from the optical transmittance and the dielectric permittivity study with variation of temperature in the range of 30 to 100 °C. Both the samples clearly show the shifting of phase transition temperature with dye concentration, especially the SmC*-SmA phase transition temperature. It is also clear from the study that SmC*-SmA phase transition phenomenon also becomes stronger with the addition of dye molecules. A theoretical explanation has also been given for shifting of phase transition temperature. The amount of shift in transition temperature agrees well as obtained from optical and dielectric studies.     

Gel–liquid crystal phase transition in dry and hydrated SOPC phospholipid studied by differential scanning calorimetry

We study the thermodynamic properties of pure and hydrated samples of SOPC (stearoyl-2-oleoyl-sn-glycero-3-phosphocholine) membrane via Differential Scanning Calorimetry. We estimate the fundamental thermodynamic quantities, such as enthalpy and entropy, of the bilayer phosphatidylcholine. It is found that the gel-liquid crystal phase transition is driven by the van’t Hoff enthalpy, revealing the occurrence of an intermediate phase transition. We discuss the influence of the heating rate on the enthalpy and on the gel ? liquid crystal phase transition temperature by introducing the adequate thermodynamic Gibbs potential. The effect of hydrogen bonding of the water molecules with the polar head and polar-apolar interface on the energetics of the bilayer membrane matrix is analysed. The obtained transition temperature was found to vary between 3 and 4°C depending on the hydration level. A result corroborated by the behaviour of the heat capacity of SOPC computed via Molecular dynamics simulation.     

Growth and dc conductivity studies of tripotassium sodium dichromate single crystal

DC electrical conductivity studies were carried out along the three crystallographic axes for tripotassium sodium dichromate (K₃Na(CrO₄)₂ or KNCr). Earlier studies of phase transition in this crystal show successive phase transitions at 239 K and 853 K. In this paper we report the dc electrical conductivity measurements in the temperature region 303–430 K along the crystallographic axes. An anomaly in conductivity was obtained around 326 K along both the axes. This may be attributed as due to a newly observed phase transition in the crystal. DSC taken for the sample also shows exothermic peak supporting the occurrence of newly observed phase transition.     

Proton magnetic resonance study of the low-temperature phase transition in a LiNH4SO4 single crystal

The proton NMR line width and spin-lattice relaxation times for LiNH₄SO₄ single crystal were studied at low temperature range of 6 and 280 K. The changes in the proton relaxation behavior near the phase transition temperature indicates a change in the state of internal motion at the transition. The molecular motions obtained by the spin-lattice relaxation processes were found to be determined by molecular reorientation of the NH₄ ions in phases III, IV, and V. We also confirmed that the phase transitions occur at 26 and 133 K.