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钙钛矿材料早期指钛酸钙(CaTiO3)矿物质,后泛指与CaTiO3具有相同结构的晶体材料,如SrTiO3、 BaTiO3、PbTiO3等[1–4],化学通式为ABO3,其中:A位元素通常为碱土元素[5–7]或者稀土元素[8–10],B位元素通常为过渡金属元素[11–13]。该类材料由于具有丰富的元素组合与结构变化,在光电[14–18]、催化[19–21]、电子器件[22–24]等领域展现出广阔的应用前景。极端高压技术在过渡金属钙钛矿材料合成及物性研究方面具有独特优势,通过调节晶体结构改变其电子结构,可实现过渡金属钙钛矿材料的热力学稳定性、磁电耦合和催化效率等性能的优化调控。近年来,愈发多样的新型过渡金属钙钛矿材料在极端高压条件下被合成,其新奇物性也被深入研究,不断促进科学与工程领域的发展。
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高压技术最初应用于地质学领域,旨在模拟地壳的高压环境。最早的高压高温合成装置是活塞圆筒型压机,由外部约束圆筒和内部液压推动的轴向活塞组成,可实现1 GPa量级的稳定压力。随着材料性能的不断提升,硬质合金逐渐取代原来的钢制结构,使得该类压机的极限压力可达5 GPa[25],如图1(a)所示。1952年,Bridgman首次提出“压砧”概念,优化了传压组件的几何结构,将其设计的Bridgman型压机的极限压力提高至10 GPa量级[26]。随后,Hall设计了两面顶Belt型压机[27],将活塞圆筒型压机的年轮式结构与Bridgman型压机的压砧结合,通过电阻加热方式首次实现了长时间且稳定的高压高温合成环境。
受机械结构和材料强度的限制,两面顶压机的极限压力难以突破10 GPa。Hall[28]在1958年设计了世界上首台多压砧压机—四面顶压机,该压机一方面增大了样品腔体积,另一方面将极限压力提高至12 GPa。但是,由于难以保证高度同步加压,四面体压砧容易造成压砧碎裂或传压介质失效。因此,这种设计并未得到广泛应用。在此基础上,Von Platen优化了多压砧压机的结构,设计了六面顶压机,如图1(b)[29]所示。六面顶压机采用前后、左右、上下3对压砧同步加压的方式,不仅可以产生10 GPa的高压条件,还能提供厘米级的压腔尺寸[30]。由于具有稳定性高、维护简单等特点,六面顶压机被广泛应用于新型钙钛矿材料的研制[31–32],以及人造金刚石、立方氮化硼等超硬材料的工业生产[33–35]。
为了进一步提升压机的极限压力,研究者基于二级传压原理设计了多面砧压机。多面砧压机首先在外部压力下挤压外层6块一级压砧,每块一级压砧的向内压缩量保持相等,形成完美的立方体压缩空间;然后通过内层8块带切角的二级压砧同时挤压中心处的八面体传压介质和样品,进而提高极限压强和传压效率。按照一级压砧的不同构型,常见的该类装置又可分为Kawai型(见图1(c)[36])、Walker型(见图1(d)[37])和DIA型(见图1(e)[38])。结合碳化钨硬质合金、聚晶金刚石等材质的二级压砧,多面砧压机目前可达到80 GPa、2 000 ℃以上的极限压力和温度[39–41]。该类压机为在更高压力区间合成新型钙钛矿材料奠定了硬件基础。
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高压原位测量装置体积小巧、结构简单,便于安装在各种测试设备上,可实现极端高压条件下样品的多种原位物性测量,但同时也对压腔的材质和结构设计提出了更高要求。根据不同的机械结构,高压原位测量装置主要分为活塞圆筒压腔 (piston-cylinder cell,PCC)、立方六面砧压腔(cubic anvil cell,CAC) 和金刚石对顶砧压腔 (diamond anvil cell,DAC)。
用于高压测量的活塞圆筒压腔的结构与大型活塞圆筒型合成压机类似,通过轴向活塞对传压介质和样品施加压力,能够产生约3 GPa的最高压强,如图2(a)[42]所示。其中,压腔外壳与活塞通常采用硬质合金材质,传压介质一般选取硅油、矿物油等液体。该类装置的样品空间可达厘米级,能够提供良好的等静压环境,便于研究带有衬底器件等大体积样品的物理特性[43–44]。
立方六面砧压腔由Mori等[45]、Cheng等[46]设计改进,具有与DIA型压机类似的导向滑块结构,形成高度对称的立方体压腔,通过驱动6个压砧从3个正交方向同步挤压中心的立方密封块获得高压条件,如图2(b)所示。这种装置可达到15 GPa的极限压力,并具有毫米级的样品空间,能够实现高压下样品的比热容、电输运、磁化率等物性测量。由于其特殊的三轴加压方式,结合液体传压介质,有利于在各向同性的等静压条件下获得样品的本征物性信息。
金刚石对顶砧压腔由一对金刚石对顶砧和密封垫组成,样品置于压腔内部,如图2(c)和图2(d)所示。因为金刚石具有极高的硬度和强度,能够承受极高的压力,通过对金刚石对顶砧之间的固体、液体或气体传压介质加压,可达地球内部几十至数百吉帕的压力范围。此外,由于金刚石对顶压砧具有透光性,并且能在几千摄氏度的高温条件下工作,因此,可用来模拟地球深部的高压高温环境,研究物质在这些极端条件下的行为和物性[47–49]。金刚石对顶砧的缺点在于样品空间非常有限,仅为几百甚至几十微米,因此,实验中对电极布置和样品装载的技术要求较高[50]。
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本课题组在高压高温合成领域利用
1 000 t Walker型二级增压装置,结合自主设计生产的高压组件耗材,通过结构和材料的优化达到了较为领先的高压高温合成条件:采用6.0 mm切角二级压砧时,达到了14 GPa的极限压强 (ZnTe金属-绝缘体转变:13.5 GPa[51],图3(a));采用2.5 mm切角二级压砧时,达到了20 GPa的极限压强(GaAs金属-绝缘体转变:18.9 GPa[51],图3(b),其中lg R 为样品电阻取对数)。对应的校压曲线如图3(c)和图3(d)所示。在高压原位测量方面,本课题组利用金刚石对顶砧压腔实现了样品的高压原位拉曼光谱、X射线衍射、电学性质、磁学性质测量,从而获得了样品在压力调控下的化学键强度变化、晶体结构相变、电子结构相变和磁相变等信息。金刚石对顶砧的砧面直径通常为300~500 μm,结合固体、液体或气体3种传压介质,利用红宝石荧光光谱标定方法和金刚石拉曼光谱标定方法,可达到的实验压力范围为0~60 GPa。金刚石对顶砧压腔实物和样品组装示意图如图4所示。
利用这些高压技术,可以合成常压条件下难以获得的新型钙钛矿材料。压力还可调节晶格结构和电子结构,进而为理解高压条件下钙钛矿的物性和相图演化规律提供新思路。本文将着重介绍过渡金属钙钛矿材料在较高压力条件下的研究进展。
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受到离子半径和化学价态等限制,常压条件下能够合成的钙钛矿材料种类有限,难以制备晶体结构高度畸变或具有非常规价态的体系。高压条件下的化学反应路径可能与常压条件不同,压力能够显著影响钙钛矿材料的晶体结构,如晶胞体积、原子间距等,可极大拓展钙钛矿结构容许的元素种类和离子失配度,进而获得更加丰富的新型钙钛矿体系。
在简单钙钛矿氧化物ABO3结构中,如图5(a)所示,A位碱土金属或稀土金属离子位于八面体间隙中,与O离子形成十二配位多面体,A位离子一般具有+2或+3价;B位过渡金属离子与周围O离子形成六配位BO6八面体,八面体通过共顶点的O离子连接,B位过渡金属离子通常具有+3或+4价[52–55]。Goldschmidt[56]提出容忍因子t (tolerance factor)以判断钙钛矿结构的稳定性,其表达式为
式中:lA―O为A―O键的键长,lB―O为B―O键的键长。
如图5(a)所示,当t = 1时,近邻B―O―B键角为180°,钙钛矿具有高度对称无畸变的立方结构;当t < 1时,A位离子半径太小,不能填满BO6八面体的空隙,共角连接的BO6八面体将发生转动或畸变以减少A―O键与B―O键的键长不匹配,致使钙钛矿形成四方、正交等对称性较低的结构;当t > 1时,由于A位离子半径较大,B位过渡金属离子将通过偏移几何中心的方式,或者BO6八面体将通过共面连接的方式,增加容纳A位离子的空间,形成三方或六方类钙钛矿结构[57]。值得注意的是,常压条件下,钙钛矿结构通常只在t接近1时才能保持稳定;高压条件下,由于压力可有效调节A―O键和B―O键的压缩或拉伸比例,钙钛矿倾向于形成对称性更高的晶体结构,因此,压力能够使容忍因子t逐渐趋近1,有利于获得稳定的钙钛矿结构。
近年来,人们在较高压力条件下通过在简单钙钛矿结构ABO3(如图5(b)所示)的基础上对A位或B位进行离子掺杂,或者利用卤族或氮族等离子替换O离子,引入尽可能多的元素种类,拓展钙钛矿结构中的离子排列构型(如图5(c)所示的A位有序双钙钛矿,其中,A'为A位引入的另一种离子)。此外,通过在八面体层间插入平面原子层,可获得Ruddlesden-Popper (RP)型层状钙钛矿材料。例如,新发现的镍基高温超导体La3Ni2O7就是具有双层八面体的RP结构[58–60],如图5(d)所示。这些钙钛矿衍生物通常呈现非常规化合价,具有不同的化学键合和电子结构特性,表现出新奇的磁性、超导电性或绝缘体-金属转变,或者在高压下进入量子临界区域,引发丰富的量子现象,如量子霍尔效应和拓扑绝缘态等[61–63]。以下将对几类新型钙钛矿体系进行详细介绍。
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PbTiO3可在常压条件下合成[64],结构容忍因子为1.02,其铁电性源于Ti4+离子偏离八面体中心而产生的自发电荷极化[65–66]。除此之外,该体系的其他化合物由于具有复杂的电子结构,均需在高压条件下才能合成,并表现出丰富的磁性、电荷转移或歧化等特性。
Shpanchenko等[67]在2004年首次发现了具有四方钙钛矿结构的PbVO3,随后Yamamoto等[68]使用六面顶压机,在6 GPa、
1173 K的高压高温条件下成功合成了立方钙钛矿PbVO3,并对其物性进行了详细研究。结果表明,PbVO3晶格中的八面体发生了显著畸变,即中心V离子严重偏离几何中心,导致其中一个V―O键断开,形成VO5四棱锥结构,如图6(a)所示。在高压条件下,PbVO₃的电阻显著下降,发生绝缘体-金属相变,其物理机制与V―O键长变化和V离子轨道的重新排列密切相关[69]。单晶样品的μ子自旋/弛豫/共振(muon spin rotation/relaxation/resonance, μSR)测试结果显示,PbVO3表现出层状反铁磁序,其转变温度在43~50 K之间[70]。Arévalo-López等[71]使用Belt型压机在8 GPa、
1073 K条件下合成了PbCrO3,并对其晶体结构和磁性进行了详细表征。X射线衍射结果表明,PbCrO3具有与PbTiO3类似的立方晶体结构。然而,衍射峰的宽化引起了人们的注意。细致的电子显微表征显示,PbCrO3实际上具有更加复杂的晶体结构,如图6(b)所示,其中Pb位和O位均存在空位。Mao-Bell型DAC的原位高压研究表明,在特定压力和温度条件下,PbCrO3发生了结构相变,并伴随着显著的体积塌陷,这种现象在钙钛矿结构材料中较为罕见,其主要驱动力源于Cr―O键的重排和Pb离子的位移[72]。磁性研究表明,PbCrO3在245 K时发生反铁磁转变。随着温度降低, Cr4+的磁序在185 K时开始重新排列,并一直持续到62 K。这种磁序的重新排列与Pb2+的最外层6s电子和Cr4+的相互作用有关[72–73]。Zhao等[74]采用高分辨X射线吸收谱的部分荧光产额模式(partial fluorescence yield X-ray absorption spectroscope,PFY XAS)结合光学DAC高压原位测试技术,对PbCrO3在高压下的价态转变进行了精确测定。通过对Pb元素L3边的PFY XAS谱及Cr元素L2,3边的软XAS谱分析得知,常压下PbCrO3中的Pb离子存在Pb2+和Pb4+两种价态,Cr离子则存在Cr3+/Cr6+电荷歧化。PbCrO3的复杂结构可通过Cr离子的电荷歧化和高价态Cr离子的四面体配位来解释,PbCrO3在高压下的绝缘体-金属转变原因则是压力对Cr离子电荷歧化的抑制。六方结构的PbMnO3可在8 GPa、
1073 K高压高温条件下合成。随后,Li等[75]使用Walker型二级增压装置,将合成条件提高至15 GPa和1273 K,成功获得了钙钛矿结构的PbMnO3。令人意外的是,尽管其容忍因子t > 1,钙钛矿型PbMnO3仍然具有非极化四方结构。如图6(c)所示,Pb和Mn离子存在复杂的电荷歧化,并未形成周期性的有序排列。物性测量结合理论计算表明,PbMnO3可能是该体系中唯一的铁磁性金属。PbFeO₃由Tsuchiya等[76]使用六面顶压机在7 GPa、
1373 K的高压高温条件下合成。随后,Ye等[77]对其物性进行了详细研究,发现在PbFeO₃中Pb和Fe离子表现出新型电荷有序现象,Pb离子表现出不同的氧化态,而Fe离子的自旋发生重新取向。这些行为具有明显的温度依赖性(图7(a),其中M为样品磁化率,T为温度,Γ1、Γ14、Γ4分别表示不同的磁对称性,CAFM代表共线反铁磁性,SR代表自旋重取向态,WFM代表弱铁磁性,PM 代表顺磁性),可能源于晶格结构在不同温度下发生的微小畸变。Fe离子之间的电子相互作用和Pb离子的价态变化诱导了复杂的电荷和自旋行为。Sakai等[78]使用Kawai型二级增压装置在12 GPa 、
1473 K的高压高温条件下合成了PbCoO3钙钛矿。Liu等[79]、Hariki等[80]、Lou等[81]对PbCoO3钙钛矿的价态和物性进行了深入研究。研究结果表明,PbCoO3中A位离子表现出电子有序行为,包括不同氧化态的分布和电子密度的重排,s-d能级之间的电子跃迁和交互作用在其中起到关键作用。DAC原位高压电学研究表明,在不同温度和压力条件下,PbCoO3中Co离子的自旋态发生由高自旋态到低自旋态的连续转变(图7(b)),表明金属间的电荷转移对材料的电导率和磁性具有重要影响。PbNiO3是一种具有复杂电子结构和磁性的过渡金属氧化物。Inaguma等[82]、Hao等[83]、Wang等[84]使用六面顶压机,在3 GPa、
1073 K的条件下合成了PbNiO3,其具有正交钙钛矿结构,并发现在常压下退火后PbNiO3会转变为极性LiNbO3型结构,如图7(c)所示,正交钙钛矿结构与LiNbO3型结构存在明显的晶格畸变和离子位置差异。物性测量结果显示,PbNiO₃在不同结构类型下具有不同的磁有序和能带结构,表明晶格结构与价态变化及电子性质存在密切联系。 -
Jin等[85]、Zhou等[86]利用六面顶压机和Kawai型二级增压装置,在高压高温条件下合成了钙钛矿ARuO3(A为碱土金属元素)体系,并对其进行了系统的物性研究。如图8(a)所示,钙钛矿CaRuO3和SrRuO3的容忍因子t < 1,具有Pbnm正交结构;利用半径更大的Ba离子替代A位Ca离子和Sr离子,将使容忍因子t趋近1,从而获得具有立方
$Pm\overline 3 m $ 结构的铁磁性金属BaRuO3,其居里温度为60 K。3种材料的粉末X射线精修结果如图8(b)所示,其中:2θ为衍射角度,I为衍射强度。Jeng等[87]、Zhong等[88]、Kojitani等[89]详细研究了CaxSr1−xRuO3和BaySr1−yRuO3固溶体的结构和物性演化规律,发现其磁性与Ru―O―Ru键角、带宽和离子键成分等密切相关,如图8(c)和图8(d)所示,其中:ρ为电阻率,χ为磁化率,S为热电势,rA为A位离子半径,Tc为磁转变温度,ZFC和FC为磁化率测试时采用的零场冷(zero field cooling,ZFC)和场冷模式(field cooling, FC),H为外磁场。这些结果为研究铁磁性相变临界行为提供了重要实验依。BaIrO3因其B位过渡金属Ir离子产生的强自旋轨道耦合相互作用表现出更加丰富的物理性质。常压合成的 BaIrO3 具有与9R相的BaRuO3相同的晶体结构,并在Walker型二级增压装置的高压下逐步经历9R→5H→6H→3C结构转变[90],如图9(a)和图9(b)所示,其中,C和F为不同朝向八面体层,C∶F为不同朝向八面体层的比例,γ为比热系数。BaIrO3的9R 相是具有180 K 居里温度的铁磁绝缘体;5H 相是具有50 K居里温度的铁磁金属;3C钙钛矿相表现出泡利顺磁性和费米液体行为;在6H 相附近,居里温度随带宽变化被逐渐抑制并接近量子临界点(quantum critical point, QCP),呈现非费米液体(non-Fermi liquid,nFL)行为[91–93],如图9(c)和9(d)所示。X 射线衍射和透射电子显微镜研究表明,压力可对 BaIrO3 的结构产生显著影响。这些实验结果为理解高压下钙钛矿材料的结构和物性演化规律提供了新的视角。
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将钙钛矿氧化物的O元素替换为卤族元素,可以获得大量的新型卤化物钙钛矿,并可能诱导其发生晶体结构相变,引起电子态密度的变化和自旋-轨道耦合效应的增强,改变带隙宽度,进而影响光吸收和发射性质,形成新的超导相或拓扑绝缘体相等。由于卤族元素原料对空气和光照敏感,在加热过程中容易分解,因此,多数卤化物钙钛矿需要在高压条件下合成。
采用Walker型二级增压装置在 16~18 GPa、
1273 ~1473 K 条件下压缩NaNiF3 钙钛矿,NaNiF3 钙钛矿会转变为后钙钛矿(post-perovskite, pPv)[94–95]。结构精修结果表明,NaNiF3 钙钛矿和后钙钛矿由于缺少Jahn-Teller 活性离子,其中的 NiF6 八面体未发生畸变,但两者的排列方式存在显著差异。NaNiF3钙钛矿在 156 K 下经历倾斜反铁磁转变,而后钙钛矿在 22 K发生反铁磁转变,这是因为后者的Ni-F-Ni 网格畸变较大,且八面体排列的维度较低(图10(a)),导致磁交换作用更小。NaCoF3在高压高温条件下也会从钙钛矿结构转变为后钙钛矿结构[96]。这种转变在地球深部条件下可能发生,这对于理解地幔中类似化合物的行为具有重要意义。不同的是,在超过26 GPa的DAC原位高压激光加热条件下,NaCoF3后钙钛矿相会分解成未知相的混合物,并且在释放压力后,这些相变成无定形态(图10(b)),而在NaNiF3中没有观察到这种不均匀分解现象(图10(c)),可能与Co2+和Ni2+的自旋态相关。NaZnF3同样在11 GPa以上发生钙钛矿-后钙钛矿的结构转变。高压拉曼光谱结合理论计算结果表明,该材料在25 GPa可能存在另一种结构相变,形成后后钙钛矿(post-post-perovskite, ppPv)相[97]。
第一性原理计算研究表明,大多数NaBF3卤化物钙钛矿与氧化物钙钛矿不同,具有名义的Born有效电荷,其铁电不稳定性源自离子尺寸效应,对压力和应变不敏感[98]。唯一例外的是NaMnF3,会在8~11 GPa的压力范围内分解为Na3Mn2F7和MnF2[99],可能具有异常极化的铁电和多铁基态。
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过度金属氧化物双钙钛矿的结构类似于简单钙钛矿,但具有更加复杂的化学式,一般为AA′B2O6、A2BB′O6或
${\mathrm{A}}{\mathrm{A}}'_3 {\mathrm{B}}_4 {\mathrm{O}}_{12}$ ,其中:A和A′表示不同的A位元素,可能具有不同的占位比例; B和B′表示不同的B位过渡金属元素。这些双钙钛矿通常表现出丰富的物理和化学性质,如磁性[100]、铁电性[101]、光学特性[102]等,在材料科学和应用领域具有重要的研究价值。CaMnTi2O6和CaFeTi2O6是少见的具有a+a+c−型Glazer八面体旋转对称性的A位有序双钙钛矿材料,前者表现出多铁特性,而后者具有非极性的四方结构(图11(a))和绝缘性(图11(b))。Li等[103]、Leinenweber等[104]利用Walker型二级增压装置在15 GPa高压条件下合成了具有钙钛矿结构的单相CaFeTi2O6,并对其物性进行了系统研究。直流磁化曲线在30 K以下出现分叉,如图11(c)所示(其中μeff为有效磁矩,Tf为铁磁转变温度),结合交流磁化率随频率的变化趋势可知,该材料可能在低温下进入自旋冻结状态。磁化率的居里-外斯拟合结果显示,CaFeTi2O6的外斯常数接近零,表明体系中的磁交换作用可能受到阻挫效应而相互抵消。随着外加磁场的升高,自旋冻结的转变温度逐渐降低,可能达到量子临界点。这为新型量子自旋液体研究提供了候选材料。
Sr2FeMoO6是一种能够在常压下合成的双钙钛矿材料,当其B位的Fe/Mo离子具有高度有序的排列方式时,材料具有410~450 K的高温本征隧穿型庞磁电阻[105]。但常压下合成的Sr2FeMoO6往往具有无序的B位离子排列,使材料无法表现出庞磁电阻性质[106]。材料磁性的X射线磁圆二色性(X-ray magnetic circular dichroism, XMCD)研究发现,B位离子的无序排列会导致材料的反铁磁有效磁矩减弱[107]。由于B位离子有序排列时Sr2FeMoO6具有最小的晶胞体积,Retuerto等[108]使用活塞圆筒型压机在2 GPa、800 ℃的条件下进行材料合成,研究发现,高压条件对于形成高度有序的双钙钛矿结构有显著的促进作用。Sr2FeReO6同样能够在常压下合成,并具有居里温度约432.3 K的高温铁磁性,材料光学性质的研究表明,其磁有序源于B位离子的3d-5d轨道电荷交换[109]。活塞圆筒压机合成的样品的测试结果表明,高压不利于形成高度有序的B位离子排列,反而会导致Fe离子无序团聚,但这种团聚使Fe离子之间形成了更强的磁耦合,增强了材料的铁磁性[110]。
在双钙钛矿ACu3Fe4O12(A = Ca,Sr,La)体系中,Kawakami等[111]、Hao等[112]、Yamada等[113]、Buitrago等[114]使用Kawai型二级增压装置,在15 GPa、
1300 K的条件下合成了A位有序CaCu3Fe4O12,并采用DAC进行了原位高压物性研究。X射线衍射、X射线吸收光谱、穆斯堡尔谱等实验结果证实CaCu3Fe4O12存在电荷歧化现象,其中,Fe4+歧化为Fe3+和Fe5+离子,并且随着压力的升高,Fe4+歧化被逐渐抑制,与此同时,伴随着Fe离子从高自旋态到低自旋态的转变以及绝缘体-金属相变,如图12(a)所示,其中p为压力。Long等[115]使用六面顶压机在10 GPa、
1400 K的高压高温条件下合成了双钙钛矿LaCu3Fe4O12,其Fe离子和Cu离子在不同温度下会发生价态变化,例如,在温度升高时,Fe 的氧化态从 Fe3+转变为 Fe2+,而 Cu 的氧化态从 Cu2+转变为 Cu3+,如图12(b)所示。这种价态变化是导致负热膨胀的主要因素之一。同时,在特定温度范围内,材料的磁结构和电子态发生变化,会进一步影响晶格结构。随后,SrCu3Fe4O12在15 GPa、1273 K的高压高温条件下被成功合成[116–119],除了具有与CaCu3Fe4O12类似的Fe离子电荷歧化特性,还在约400 K时表现出巨大的负热膨胀效应,其负热膨胀系数α = −2.26×10−5 K−1,如图12(c)所示,这与Fe-Cu离子之间的电荷转移密切相关。CaCu3Ti4O12是极少数能够在常压下合成的
${\mathrm{AA}}'_3 {\mathrm{B}}_4 {\mathrm{O}}_{12} $ 型双钙钛矿材料,具有巨介电常数和极低的损耗[120],并在约25 K以下表现出反铁磁性[121],因此,能够广泛应用于介电、压电等领域[122]。若将Ca离子替换为同主族的Sr离子,A位离子半径增大导致的晶格畸变使得SrCu3Ti4O12需要使用二级压机在7.7 GPa的高压下才能合成[123]。SrCu3Ti4O12同样在低温下具有反铁磁性,其物理机制与CaCu3Ti4O12相同,即Cu离子以Ti离子为中介的间接交换作用与自旋-轨道耦合作用共存。然而,高压合成的SrCu3Ti4O12中,Cu占位存在缺陷,因此,并未观察到巨介电常数效应。 -
在高压条件下合成的钙钛矿及其衍生材料往往展现出独特的晶格结构和物理特性,一直是物理、化学和材料领域研究的前沿焦点。具体而言,高压技术具有以下独特优势:
(1) 在高压条件下,钙钛矿材料的结构畸变减小,有利于形成高对称性的晶体结构,通过改变离子间距使材料的容忍因子更接近1,可以得到常压下无法形成的新型钙钛矿材料;
(2) 高压合成提供高度密封环境,能够有效防止加热过程中易挥发元素的逸散,保证严格的化学计量比,特别有利于制备成分复杂的钙钛矿材料;
(3) 高压可提供化学反应驱动力,通过改变化学反应途径并缩短化学反应时间提高样品合成效率;
(4) 高压易与极低温、强磁场、超快光场等极端条件相结合,可实现钙钛矿材料的磁电、超导、拓扑等性质的综合调控,在凝聚态物理研究领域具有重要意义。
另一方面,高压技术也存在一定的局限性,主要包括:
(1) 高压设备通常昂贵且复杂,需要精密的设计和制造,且维护和操作需要专业技术支持,增加了研究成本;
(2) 高压实验通常在微小样品上完成,实验难度较高且具有偶然性,此外,样品在高压环境中可能会发生形变或化学反应,影响实验结果的可靠性;
(3) 高压实验需要精确控制压力和温度,不同实验室在相同条件下也可能得到不同的结果,对科学研究的可信度和有效性提出了挑战;
(4) 高压能够通过调控原子间距,改变材料中离子轨道交叠程度和能带结构,因而,材料在高压下的结构、组分和价态与常压下的情况可能完全不同,会产生不同的物理、化学性质,因此,需要不断发展新的理论模型和方法来分析高压数据。
综上所述,高压技术将钙钛矿材料研究拓展至新的维度,使研究人员能够在极端条件下深入探索结构与物性之间的内在联系,推动了新材料的研发与应用,为研制新型量子器件、推动发展新质生产力奠定了技术基础。
过渡金属钙钛矿的高压高温合成及物性
High Pressure High Temperature Synthesis and Physical Properties of Transition Metal Perovskites
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摘要: 过渡金属钙钛矿材料由于具有灵活多变的晶体结构和丰富多样的物理性质,在信息、能源和催化等领域具有广阔的应用前景。然而,在常规条件下合成的过渡金属钙钛矿种类有限。高压作为一种独特的实验手段,能够显著调控材料的原子间距和元素构型,在合成新型钙钛矿材料方面具有较大优势,通过改变电子结构可引发铁电、磁性、超导、金属-绝缘体转变、电荷转移及电荷歧化等新奇的物理性质。本文回顾了极端高压材料制备技术和高压原位测量技术,并对这2项技术在几类过渡金属钙钛矿合成与物性调控方面的应用进行了展望。Abstract: Transition metal perovskite materials hold broad prospects for applications in fields such as information technology, energy, and catalysis due to their flexible and diverse crystal structures and rich variety of physical properties. However, the types of transition metal perovskite materials synthesized under conventional conditions are limited. High pressure, as a unique experimental approach, can significantly manipulate atomic distances and elemental configurations in materials. This method offers substantial advantages in synthesizing novel perovskite materials and can induce novel physical properties such as ferroelectricity, magnetism, superconductivity, metal-insulator transition, charge transfer and charge disproportionation by altering electronic structures. In this paper, the preparation of extreme high-pressure materials and high-pressure in-situ measurement techniques, as well as their applications in the synthesis and physical properties control of several types of transition metal perovskite materials are reviewed.
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Key words:
- extreme high pressure /
- transition metal perovskites /
- crystal structure /
- physical properties .
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图 1 高压高温合成装置:(a) 活塞圆筒型压机[25],( b) 六面顶压机[29] ,(c) Kawai型二级增压装置[36](F为外部压力),(d) Walker型二级增压装置[37] (U为活塞压力),(e) DIA型二级增压装置[38]
Figure 1. High-pressure and high-temperature apparatus: (a) piston-cylinder press[25]; (b) cubic press[29]; (c) Kawai-type muti-anvil module[36] (F being outer force); (d) Walker-type muti-anvil module[37] (U being piston pressure); (c) DIA-type muti-anvil module[38]
图 2 高压原位测量装置:(a) 活塞圆筒压腔[42] ,(b) 立方体压腔[45],(c) 用于电学测量的金刚石对顶砧[47],(d) 用于单晶X射线衍射测量的金刚石对顶砧[47]
Figure 2. In-situ high-pressure measurement devices: (a) piston-cylinder cell[42]; (b) cubic anvil cell [45]; (c) diamond anvil cell for electrical measurements[47]; (d) diamond anvil cell for single crystal X-ray diffraction measurements[47]
图 3
1000 t Walker型二级增压装置及校压结果:(a) 6.0 mm切角二级压砧ZnTe的压力-电阻曲线, (b) 2.5 mm切角二级压砧GaAs的压力-电阻曲线,(c) 6.0 mm切角二级压砧校压结果,(d) 2.5 mm切角二级压砧校压结果Figure 3. Pressure calibration of
1000 t Walker-type apparatus: (a) ZnTe resistivity-pressure curve using 6.0 mm edge lengthsecond stage anvil; (b) GaAs resistivity-pressure curve using 2.5 mm edge length second stage anvil; (c) pressure calibration result using 6.0 mm edge length second stage anvil; (d) pressure calibration result using 2.5 mm edge length second stage anvil图 6 (a) PbVO3的结构畸变和轨道杂化[70],(b) PbCrO3的复杂晶体结构[71] (其中,黄球为Pb离子,其大小表示填充率高低;紫球为Cr离子;红球为O离子 ),(c) PbMnO3的电荷歧化[75]
Figure 6. (a) Structural distortion and orbital hybridization of PbVO3[70]; (b) complex crystal structure of PbCrO3[71] (Yellow ball is Pb ions, the size of which indicates the filling ratio; purple ball is Cr ions and red ball is O ions.); (c) charge disproportionation of PbMnO3[75]
图 8 (a) ARuO3体系晶体结构示意图[85],(b) ARuO3体系粉末X射线衍射精修结果[85],(c) BaRuO3的物性测试结果[85],(d) Ca/Ba掺杂的SrRuO3体系的结构和物性总结[85](TG为Griffiths相温度,O代表正交结构,T代表四方结构,C代表立方结构)
Figure 8. (a) Crystal structure of ARuO3 system[85] ; (b) powder XRD refinement result of ARuO3 system[85]; (c) physical properties of BaRuO3[85]; (d) summary of crystal structure and physical properties of Ca/Ba doped SrRuO3 system[85] (TG is Griffiths phase temperature, O stands for orthorhombic structure, T stands for tetragonal structure and C stands for cubic structure.)
图 9 (a) BaIrO3随着压力升高发生的结构转变[91],(b) 不同结构的BaIrO3的物性演变[91],(c) BaIrO3的电阻率及磁化率随温度的变化曲线[91],(d) 不同结构BaIrO3和BaRuO3的磁性[91]
Figure 9. (a) Structural transition of BaIrO3 with increasing pressure[91]; (b) evolution of physical properties of BaIrO3 with various structures[91]; (c) temperature dependent resistivity and magnetic susceptibility of BaIrO3[91]; (d) magnetic susceptibility of BaIrO3 and BaRuO3 polytypes[91]
图 10 (a) NaBF3(B = Mg, Co, Ni, Zn)体系八面体倾角Φ随压力的变化[97] ,(b) NaCoF3的原位高压XRD谱,(c) NaNiF3的原位高压XRD结果[97](a、b、c为晶格常数)
Figure 10. (a) Pressure dependent of octahedral tilt angle Φ of NaBF3 (B = Mg, Co, Ni, Zn) system[97]; (b) in-situ high pressure XRD patterns of NaCoF3; (c) in-situ high pressure XRD results of NaNiF3[97] (a, b, and c are lattice constants.)
图 12 ACu3Fe4O12体系的新奇物性:(a) CaCu3Fe4O12在高压下的复杂价态和物性转变[111],(b) SrCu3Fe4O12的新奇离子价态[115],(c) LaCu3Fe4O12的巨大负热膨胀系数[116]
Figure 12. Novel physical properties of ACu3Fe4O12 system: (a) complex transition of valence state and physical properties of CaCu3Fe4O12 with increasing pressure[111]; (b) strange ironic valence state in LaCu3Fe4O12[115]; (c) giant negative thermal expansion coefficient of SrCu3Fe4O12[116]
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