Intrinsic Zero-Linear and Zero-Area Compressibilities over an Ultrawide Pressure Range within a Gear-Spring Structure

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Metadata	Details
Publication Date	2022-01-17
Journal	CCS Chemistry
Authors	Dequan Jiang, Ting Bin Wen, Huimin Song, Zimin Jiang, Chen Li
Institutions	Peking University, Materials Science & Engineering
Citations	21

Abstract

Open AccessCCS ChemistryCOMMUNICATION3 Oct 2022Intrinsic Zero-Linear and Zero-Area Compressibilities over an Ultrawide Pressure Range within a Gear-Spring Structure Dequan Jiang, Ting Wen, Huimin Song, Zimin Jiang, Chen Li, Ke Liu, Wenge Yang, Ho-kwang Mao and Yonggang Wang Dequan Jiang Center for High Pressure Science and Technology Advanced Research (HPSTAR), Beijing 100094 , Ting Wen *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Center for High Pressure Science and Technology Advanced Research (HPSTAR), Beijing 100094 , Huimin Song School of Materials Science and Engineering, Peking University, Beijing 100871 , Zimin Jiang Center for High Pressure Science and Technology Advanced Research (HPSTAR), Beijing 100094 , Chen Li Center for High Pressure Science and Technology Advanced Research (HPSTAR), Beijing 100094 , Ke Liu Center for High Pressure Science and Technology Advanced Research (HPSTAR), Beijing 100094 , Wenge Yang Center for High Pressure Science and Technology Advanced Research (HPSTAR), Beijing 100094 , Ho-kwang Mao Center for High Pressure Science and Technology Advanced Research (HPSTAR), Beijing 100094 and Yonggang Wang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Center for High Pressure Science and Technology Advanced Research (HPSTAR), Beijing 100094 https://doi.org/10.31635/ccschem.022.202101739 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Materials with zero-linear compressibility (ZLC) and zero-area compressibility (ZAC) have great promise for specific applications retaining constancy in specific directions or planes under external impaction. To date, no more than 10 ZLC/ZAC materials have been reported, most of which have very limited working pressure ranges (<10 GPa). Herein, we report the observation of ZLC and ZAC in Li2Ti(IO3)6 with a gear-spring type structure over an ultrawide pressure range (0-40 GPa). Structural analysis from experimental and theoretical calculations revealed that the rotatable metal coordination polyhedral (gears) and extremely compressible metal chains (springs) work together to form an exquisite mechanical unloading device with intrinsic ZLC and ZAC behavior. Moreover, Li2Ti(IO3)6 set a record-wide ZLC/ZAC working pressure range (up to 40 GPa) among the currently known anisotropic compression materials. The demonstration of intrinsic and long-lasting ZLC/ZAC with a gear-spring mechanism allowed shock-resistant precision optics to be applied under extreme conditions. Download figure Download PowerPoint Introduction Substances typically contract under external hydrostatic pressure, just as they expand with heat and contract with cold.1-3 From a thermodynamics perspective, it is impossible for a given material to exhibit negative volume compressibility.4 Nevertheless, several material types possess negative compressibility in specific crystalline directions or planes, while the overall cell volume keeps decreasing under compression.5-9 In the last decade, materials with negative linear compressibility (NLC) and negative area compressibility (NAC) have been explored widely with specific mechanisms such as wine-rack, honeycomb networks, and the lifshitz model.10-13 In general, zero-linear compressibility (ZLC) and zero-area compressibility (ZAC) could be achieved readily by finding a balance between negative and positive compressibilities. The linear and area compressibilities of diamond and Os are used as the standard to classify a ZLC or ZAC material.14-17 ZLC and ZAC materials are ideal candidates for precision instruments applied under extreme environments such as submarine fiber-optic communication and shock-resistant optical windows.1,18,19 Surprisingly, materials with ZLC or ZAC have been rarely discovered compared with those with NLC and NAC properties.12,14,20-23 Meanwhile, there is no clear structure-property mechanism for the rational design of ZLC/ZAC materials. In the decade’s exploration of materials with abnormal compressibility, remarkable findings have been concentrated in two types of materials: metal-organic frameworks (MOFs) and all-inorganic frameworks.5 The most important strategy to achieve materials with abnormal compressibility is to establish a clear and instructive structure-property relationship. The diverse organic units endow MOFs with the ability to exhibit flexible mechanical responses under compression,24-30 including both large NLC and near-ZLC. The wine-rack structure model is the most popular mechanism describing the NLC and near-ZLC behaviors in MOFs. Particularly, the “molecular gears and torsion springs” structure model was proposed to describe the NLC and extreme compressibility of LnFe(CN)6 (Ln = Ho, Lu, or Y), which benefitted from the LnN6 torsion springs and the rigid Fe(CN)6 gears.31 However, MOFs are soft and usually show the above-mentioned abnormal compressibility within a very narrow pressure range, typically of no more than 10 GPa, which significantly limits their potential applications. Comparatively, all-inorganic frameworks possessing covalently-bonded frameworks are less compressible and include borates. Although the highest pressure of ZLC is 8 GPa, found in (Ca,Sr)B2O4 to date, from a structural chemistry viewpoint, they are still good candidates with abnormal compressibility in a relatively wide pressure range.5 In addition, all-inorganic frameworks could possess good chemical/physical stability and excellent optical performances, making them a potential for next-generation shock-resistant optical windows and fiber communications. Unfortunately, a precise mechanism for the rational design of all-inorganic frameworks with abnormal compressibility is lacking. An exception is the recently reported Lu-Ban stool model for (Ca,Sr)B2O4, in which the subtle counterbalance originated from the expansion and contraction effect between the rotation of the [BO3] “planks” and the shrinkage of Ca-O “legs” attributed to an excellent ZLC property.14 We aimed to explore all-inorganic optical materials with both excellent ZLC/ZAC properties and good chemical/physical stabilities using a structural design strategy. Numerous known inorganic compounds in the inorganic crystal structure database (ICSD) have been screened to search for framework-like structures, among which a unique family of transition metal iodates with metal-metal bonding and chains were identified with potential for ZLC/ZAC behaviors. Herein, we report the anomalous mechanical responses of Li2Ti(IO3)6 to external pressure as a representative inorganic framework structure. The anisotropic compressibility of Li2Ti(IO3)6 was examined using in situ X-ray diffraction (XRD), Raman spectra, and first-principles calculations. A gear-spring mechanism was proposed to be responsible for the emerging intrinsic ZLC/ZAC properties, which sheds light on the future structural design of all-inorganic materials with anomalous compressibility. Results and Discussion Phase-pure Li2Ti(IO3)6 was obtained using a hydrothermal method as both white powders and single transparent crystals. At ambient conditions, Li2Ti(IO3)6 crystallized in the hexagonal space group P63 with cell parameters of a = 9.372(2) Å and c = 5.113(1) Å ( Supporting Information Figure S1). Figure 1a shows the crystal structure of Li2Ti(IO3)6 consisting of face-sharing TiO6 chains along the c-axis, distorted IO3 units, and LiO6 octahedra. The structure is noncentrosymmetric with a very strong second-harmonic generation (SHG) efficiency of ∼500× α-SiO2, comparable with those of BaTiO3 (400× α-SiO2) and LiNbO3 (600× α-SiO2).32 Both the Ti4+ and I5+ cations were located within asymmetric coordination environments due to the second-order Jahn-Teller effects. The parallel alignment of the lone pair electrons on the I5+ cations contributed mainly to the macroscopic polarity of Li2Ti(IO3)6 ( Supporting Information Figure S2).33,34 Notably, each TiO6 octahedron linked with six IO3 polyhedra to form a large anionic [(TiO6/2)2−·6(IO1/2O2/1)0]2− “cylinder” in the ab plane; these nano-cylinders were separated by six-coordinated Li+ cations to achieve charge balance. In such an inorganic framework, the “cylinders” were expected to have the ability to “rotate” around the Ti-Ti axis since the connection manner between (TiO6/2)2− and six (IO1/2O2/1)0 was corner-sharing. Thus, the anionic [(TiO6/2)2−·6(IO1/2O2/1)0]2− was considered a rigid “roller.” Previous studies provided preliminary verification of anomalous compressibility of similar metal iodates, such as Fe(IO3)3 and Zn(IO3)2.35,36 Therefore, we have sufficient justification to expect Li2Ti(IO3)6 to have peculiar mechanical responses to external pressure. Figure 1 | Crystal structure and compression behavior of Li2Ti(IO3)6. (a) Hexagonal structure of Li2Ti(IO3)6 at ambient conditions, consisting of face-sharing TiO6 chain (dark gray) along the c axis, distorted IO3 units (light gray), and LiO6 octahedra (blue). (b) XRD patterns of Li2Ti(IO3)6 at high pressures. The anisotropic compressibility is evident without a structural phase transition. (c) Cell parameters and volume as pressure functions, showing intrinsic ZLC and ZAC behavior along the a/b directions and in the ab plane, respectively. The dotted lines represent the change within a very small range (±1%). (d) Pressure-dependent linear compressibilities (Ka/b and Kc) and error bars of Li2Ti(IO3)6 are calculated using the online PASCal package.37 Download figure Download PowerPoint In situ high-pressure XRD measurements were conducted to study the compressibility of Li2Ti(IO3)6 with the “inorganic-framework” structure (Figure 1b). No obvious structural phase transition was observed under compression up to 40.2 GPa since there was only a peak shift instead of a new peak emerging or disappearing, and the pressure effect on the crystal structure was reversible ( Supporting Information Figure S3). Different Bragg peaks exhibited distinct shift behaviors under compression, implying extremely anisotropic compressibility along different crystallographic axes. Among them, the (hk0) peaks such as (2 1 ¯ 0) and (300) virtually sustained their 2θ values, indicating incompressibility or minimal compressibility along the a and b axes. Specifically, the (hk0) peaks shifted slightly to the lower 2θ angle first in the 0-8 GPa range (i), and then slightly to the higher 2θ angle in the 8-19 GPa (ii), and finally to the lower 2θ angle again above 19 GPa (iii) ( Supporting Information Figure S4). By contrast, the (hkl) peaks such as (2 1 ¯ 1) with l ≠ 0 shifted quickly to the higher 2θ angle under compression up to 40 GPa, indicating large compressibility along the c axis. Figure 1c shows the refined cell parameters of Li2Ti(IO3)6 as functions of applied pressure ( Supporting Information Table S1 and Figures S5 and S6). An extremely anisotropic compression behavior was evident. The evolution of the a axis underwent three pressure regions in accordance with the peak shift of the (hk0) planes. Notably, the overall change of the a axis (Δa) in the 0-40.2 GPa region was as low as 0.97%, a low enough value to be regarded as ZLC. Due to the hexagonal symmetry of space group P63 with a = b, the ab plane exhibited similar compression behavior with individual a and b axes, that is, the overall area change of the ab plane (Δs) in the 0-40.2 GPa was as low as 1.95%, also low enough value to represent a ZAC. Comparatively, the c axis exhibited tremendous compressibility with Δc = 28.69 % at 40.2 GPa, and a normal V-P curve was obtained in the whole pressure range. The linear compressibility (Kl) has been defined previously as −dl/ldp to evaluate the strength of a given material’s mechanical response to external pressure.5 Figure 1d represents the pressure-dependent linear compressibilities (Ka/b and Kc) of Li2Ti(IO3)6, derived using the online PASCal package37 and within the three pressure ranges discussed above (i, ii, and iii). It is interesting to observe that Ka/b maintained very small values throughout the whole pressure range: The calculated values for individual pressure ranges were: Ka = 1.06 TPa−1 and Kc = 10.54 TPa−1 for (i), (0-8 GPa); Ka = −1.88 TPa−1 and Kc = 14.84 TPa−1 for (ii), (8-19 GPa); Ka = 0.58 TPa−1 and Kc = 1.78 TPa−1 for (iii), (19-40.2 GPa), respectively ( Supporting Information Figure S7). It was obvious that the Ka values in regions (i) and (iii) were both small and could be compared to those of diamond (0.75 TPa−1) and Os (0.70 TPa−1), which were deemed the standards of ZLC materials. The absolute Ka value in the region (ii) was more than twice that of diamond and Os and could be deemed NLC. However, this NLC effect was still relatively weak among the reported NLC materials,5 in which the largest one could achieve −76 TPa−1 in Ag3[Co(CN)6].11 Nevertheless, region (ii) played a significant role in averaging the positive linear compressibility (PLC) in regions (i) and (iii), and thus, achieved a ZLC within the whole pressure region. If we took the (i) + (ii) regions (0-19 GPa) into account, the positive Ka in range (i) and the negative Ka in range (ii) yielded a very small overall Ka value of −0.23 TPa−1 in the 0-19 GPa region, still small enough to be characteristic of ZLC. It was necessary to consider the distinct compression properties in both the separate and overall pressure regions for such material with “switching” compressibilities. Thus, Li2Ti(IO3)6 was a promising ZLC material in both the a and b directions in the pressure range of 0-40.2 GPa. Of course, Li2Ti(IO3)6 was also a promising ZAC material since the area change of the ab plane (Δs) was as low as 1.95% within 0-40.2 GPa. The online PASCal package is only applicable to those cases with either a positive or a negative Kl. To calculate the total Kl in our case with both positive and negative linear compressibilities in a pressure range, we simply use the most primitive formulas (Kl = −Δl/lΔp and Ks = −Δs/sΔp) to evaluate the linear and area compressibilities, where l represented the length of the starting unit cell, s represented the area of the initial crystal plane, Δl, Δs, and Δp represented the changes of linear, area, and pressure, respectively. The recalculated Kl and Ks in the pressure range of 0-40.2 GPa are provided in Supporting Information Figure S8, and the compressive properties of representative ZLC/ZAC materials, including the superhard diamond and Os, used as references, are concluded in Table 1. Table 1 | The Compressibility and Pressure Range in All Reported ZAC/ZLC Materials Material Mechanism Kl (TPa−1)a Ks (TPa−1)a P (GPa) References MIL-122(In) Wine-rack 0 / 0-10.5 22 [C(NH2)3][Cd(HCOO)3] Wine-rack 0.75 1.51 0-0.5 21 SrB2O4 Lu-Ban stool −0.47 / 0-8.0 14 CaB2O4 Lu-Ban stool 0.34 / 0-8.1 14 LiBO2 Corrugated-graphite 1.54 / 0-2.5 20 RbBe2BO3F2 Lifshitz 0.05 0.1 0-2.6 12 CsBe2BO3F2 Lifshitz −0.03 −0.07 0-3.2 12 GeO2 / 0.08 / 0-8.0 23 Dimond Superhard 0.62 1.23 0-45.0 15 Os Superhard 0.60a 1.02 0∼63.8 17 0.54c Li2Ti(IO3)6 Gear-spring 1.50 3.00 0-8.0 This work −0.13 −0.26 0-19.0 0.24 0.48 0-40.2 aKl and Ks are calculated by the formulas (Kl = −Δl/lΔp and Ks = −Δs/sΔp); a and c represent the linear compressibilities along the a and c direction. The bold values represent that those values are from this work. First of all, Li2Ti(IO3)6 possessed relatively small Kl (0.24 TPa−1) and Ks (0.48 TPa−1) within these materials, including diamond (Kl = 0.62 TPa−1 and Ks = 1.23 TPa−1) and Os metal (Kl = 0.54/0.60 TPa−1 and Ks = 1.02 TPa−1), making it worthy of a ZLC and ZAC material. Remarkably, inorganic ceramics or crystalline materials with both ZAC properties and transparency with important wavelengths have rarely been reported, holding a great promise for practical shock-resistant optical usages. Second, Li2Ti(IO3)6 exhibited ZLC and ZAC in an ultrawide pressure range, that is, 0-40.2 GPa, compared with ZLC/ZAC MOFs and metal borates. Although 40.2 GPa might not be the highest pressure value for Li2Ti(IO3)6 to survive, this value still refreshed the world record of ZLC/ZAC material (except diamond and Os). As we already know, there are several structural mechanisms for previously discovered ZLC/ZAC materials, including wine-rack for MOFs, Lu-Ban stool, corrugated graphite, lifshitz for metal borates, and superhard for diamond and Os; however, Li2Ti(IO3)6 does not belong to any of these mechanisms. The crystal structure of Li2Ti(IO3)6 could be regarded vividly as a gear-spring type. As shown in Figure 2a, the visualized gear-spring model of Li2Ti(IO3)6 comprised of Gear-A (the anionic [(TiO6/2)2−·6(IO1/2O2/1)0]2−), Gear-B (LiO6 octahedra), and an extremely compressible spring (Ti-Ti distances or c), as shown in Supporting Information Figure S9. This model was derived entirely from the real crystal structure and could perfectly describe and explain the ZLC/ZAC phenomena of Li2Ti(IO3)6. Figure 2 | The Gear-spring mechanism for ZLC/ZAC under compression. (a) Visualized gear-spring structure of Li2Ti(IO3)6, comprising of Gear-A (the anionic [(TiO6/2)2−·6(IO1/2O2/1)0]2−), Gear-B (LiO6 octahedra), and the extremely compressible spring-length (Ti-Ti distances or c). (b) Evolutions of the size and length of Gear-A, Gear-B, and spring under compression. The “gears” are expected to keep their sizes, and the “springs” are expected to exhibit extremely large compressibility. (c) Pressure-dependent rotations of Gear-A and Gear-B. ”+” indicates anticlockwise, and ”-” indicates clockwise. (d) Representative structures of the TiO6− chain at 0 and 40.2 GPa, showing the working mechanism of the “spring.” (e and f) Bond-length and bond-angle evolutions in the gear-spring structure of Li2Ti(IO3)6 under compression. Download figure Download PowerPoint The parameters of the gear-spring model, gear radii [r(Gear-A) and r(Gear-B)], spring length (c) and rotation angles [θ(Gear-A), and θ(Gear-B)] were derived from structural optimization of first-principles calculation using the experimental cell parameters of Li2Ti(IO3)6 under various pressures. As rigid “gears,” the sizes of “Gear-A” and “Gear-B” essentially remained unchanged, which was validated perfectly by the theoretical calculation results (Figure 2b). In the meantime, the spring length (c) decreased rapidly along with an increase in applied pressure, working together as real springs. Amazingly, the two gears (A and B) could rotate synergistically in terms of either direction or angles (Figure 2c). Unlike the previously derived three pressure regions (i, ii, and iii) of the cell parameter evolution, four rotation processes were observed under compression: (i) A+(counterclockwise)B− (clockwise) in 0-9 GPa, (ii-a) A+B+ in 9-14 GPa, (ii-b) A−B+ in 14-18 GPa, and (iii) A0B0 in 18-40 GPa, respectively. Thus, it was reasonable to believe that the rotation of the gears could convert the external pressure to the compression direction of the spring. Figure 2d shows the representative structures of the TiO6-chain at 0 GPa and the highest pressure of 40.2 GPa. The working mechanism of the “spring” was clear from the dramatic changes of the Ti-Ti distances and Ti-O-Ti bond angles. The c value (spring length) was reduced from 5.07 to 3.63 Å, together with the highly flattened Ti-O-Ti angle (from 81.3° to 52.4°) but almost unchanged Ti-O bondlengths (from 1.92/2.01 to 2.04/2.07 Å). The detailed changes of individual bondlengths during the gear-spring working process under compression are displayed in Figures 2e and 2f. Both the Ti-Ti distances and Ti-O-Ti angles exhibited similar changes under compression up to 40 GPa, and there was no distortion from the Ti-O bondlengths within the TiO6 octahedra. The average bondlengths of I-O and Ti-O also showed cooperative changes under high pressure to maintain the unchanged size of Gear-A. In brief, the emerging ZLC/ZAC phenomena in Li2Ti(IO3)6 could be explained accurately employing the gear-spring mechanism. Materials with such a “gear” and “spring” cooperative structure are expected to have intrinsic ZLC/ZAC properties, also in an ultrawide pressure range. We probed the local structure evolution of Li2Ti(IO3)6 during the ZLC/ZAC processes by in situ high-pressure Raman spectra ( Supporting Information Figure S10). At first, the Raman profile remained largely unchanged under compression, except for gradual shifts of the peaks, indicating the absence of a structural phase transition. The peaks located in the ranges of 400-600 and 600-850 cm−1 could be assigned to the stretching vibration modes of the [TiO6] octahedra and [IO3] pyramids, respectively. The softening of the [IO3] peaks (shifting continuously to a lower wavenumber) indicated an elongation of the I-O bonds. SHG is a common nonlinear optical phenomenon highly related to a crystal structure symmetry38-40; thus, an ideal probe to subtle structure evolution. Our high-pressure SHG experiment indicated that Li2Ti(IO3)6 remained SHG-active up to 40.2 GPa ( Supporting Information Figure S11). It was interesting to find that the trend of SHG intensity was similar to that of the cell parameter c. The SHG response of Li2Ti(IO3)6 was dominated by a consistent arrangement of lone-pair electrons along the c axis direction. Therefore, the changing trend of SHG response also reflected the compression trend of cell parameter c to some extent. The long-lasting SHG functionality combined with intrinsic ZLC/ZAC properties within an ultrawide pressure range made Li2Ti(IO3)6 a unique candidate for optically sensitive devices under extreme conditions. We envisaged a great potential to explore more ZLC/ZAC materials under the guidance of the as-proposed gear-spring mechanism: First, a large family of metal iodates exists with the general formula A2M(IO3)6 or M(IO3)n (A is a monovalent cation; M is a transition metal).35,36 It was easy to find gear-spring type isostructures with rigid face-sharing MO6 chains and flexible IO3 linkages similar to hexagonal Li2Ti(IO3)6. Also, we found it interesting to discuss the relationship between crystal symmetry and anomalous compression behaviors (ZLC/ZAC). Generally, ZLC in a = b crystalline system (e.g., hexagonal for Li2Ti(IO3)6) automatically gives rise to ZAC; therefore, compounds with high crystalline symmetries such as cubic, hexagonal, and tetragonal are preferred candidates for ZAC behavior. Theoretically, ZAC could also be achieved by combining and NLC in an a ≠ b crystalline system this has to be Second, gear-spring structural types of iodates was face-sharing octahedra could as both the springs and the large rigid and small such as and could be to the system flexible and An structure was also to exhibit ZAC behavior the gear-spring mechanism. combining the different mechanisms the rational design of more ZLC/ZAC material applicable to an ultrawide pressure range. We report intrinsic ZLC and ZAC properties in an all-inorganic material Li2Ti(IO3)6. The ultrawide working pressure range GPa) of Li2Ti(IO3)6 set a record for ZLC or ZAC materials (except diamond and Os). on structural we proposed a promising gear-spring mechanism that could perfectly describe and explain the anomalous compression behavior of Li2Ti(IO3)6. We are to similar anomalous compression properties intrinsic ZLC and in compounds with such gear-spring Moreover, Li2Ti(IO3)6 maintained nonlinear optical functionality and ZLC/ZAC behaviors to high pressure, making it promising for optical devices under extreme Supporting Information Supporting Information is and experimental and of is no of to Research in this was on a to in The be found using the This work was by the of the Science of the Science of and the of XRD were at of Beijing The for with References 1. that with a Li or of with Compressibilities in or Compressibility and in Wang Compressibility in from of Materials with in Wang Yang Compressibility in with Compressibility in Compressibility of Jiang Li Wang Li Li Li Chen Compressibility over Pressure Range in and in Compressibility in Jiang Yang Wang Liu Li Li Li Compressibility in with a of under to the of An for in Materials and from the Jiang Wang Li Li Li Materials Compressibility into Wang Compressibility in a Chen Jiang Wang Compressibility in Materials of and at High of in A and with a in a Wang Li Compressibility due to in a Wang Compressibility to Pressure in Materials Compressibility in LnFe(CN)6 Materials and of in a and of Mao on Materials on Chen Mao Yang Mao and SHG Materials the by High Pressure in and of and A for and Compressibility of Chen and on Li Yang Chen of Ultrawide Previous Information pressure work was by the of the Science of the Science of and the of XRD were at of Beijing The for with

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DOI: https://doi.org/10.31635/ccschem.022.202101739