Transformation Of Surface Structure Research Articles

Modulating Confined Interlayer Environment By Hard Lewis Acid Cation for Enhancing Oxygen Evolution Reaction

With the increasing demand for hydrogen energy, the non-noble metal catalysts in water splitting for hydrogen production are significant for addressing global environmental issues.[1] Among them, layered transition metal oxide/hydroxide (LTMO/LTMH) materials have drawn more attention for their high tunability of surface or bulk structural transformations after introducing different cations inside the interlayer, which could manifest OER activity.[2] Therefore, birnessite-layered manganese dioxide (MnO2) is considered a potential material for its ability to create a container for cation intercalation, which could significantly enhance its behavior in the oxygen evolution reaction (OER).[3] However, most of the investigations of its application in OER considered MnO2 layers as active sites, which may underestimate the function of intercalated cations and hence blur its real reaction active sites. Using the hard and soft acids and bases (HSAB) theory recently sheds new light and provides a new road for improving OER performance; hard Lewis acid captures hydroxyl and thus speeds up the cleavage of water molecules, generating a demanding local alkaline environment.[4-6] Herein, we grounded the perspective of the intercalated cations based on the HSAB theory, using layered MnO2 as the “container” for accommodating hard Lewis acid cations, achieving using Lewis acid cations for active sites, to enhance OER activity. We use the electrodeposition method to synthesize the precursor layered catalyst, TBA+/MnO2. With the ion exchange process, cations with different Lewis acidities (K+, Fe3+, Co2+, Ni2+, Cu2+, and Zn2+) were intercalated inside this “nano-cell” (interlayer of layered MnO2) to discuss how they influenced the confined surrounding electrolyte environment and electron/mass transport. X-ray diffraction, Raman, and inductively coupled plasma verified the successful synthesis of layered MnO2 with different cations intercalated. The OER results of cation-intercalated MnO2 catalysts exhibited that intercalating the harder Lewis acid significantly influences the interlayer electrolyte environment, markedly accelerating and enhancing the reaction kinetics and OER activity. Furthermore, comparing the OER behavior of intercalated cations, free cations, and bulk metal foils, the intercalated hard Lewis acid cations showed superior reaction kinetics. This work thus provides a clear picture of the role of cations when intercalated into the MnO2 layers during OER and highlights a novel utilization of Lewis acid cations as active sites for enhancing OER activity. Reference [1] Wang, S.; Lu, A.; Zhong, C.-J. Hydrogen Production from Water Electrolysis: Role of Catalysts. Nano Converg. 2021, 8 (1), 4.[2] Kim, Y.; Choi, E.; Kim, S.; Byon, H. R. Layered Transition Metal Oxides (LTMO) for Oxygen Evolution Reactions and Aqueous Li-Ion Batteries. Chem. Sci. 2023, 14 (39), 10644–10663.[3] Ju, M.; Chen, Z.; Zhu, H.; Cai, R.; Lin, Z.; Chen, Y.; Wang, Y.; Gao, J.; Long, X.; Yang, S. Fe(III) Docking-Activated Sites in Layered Birnessite for Efficient Water Oxidation. J. Am. Chem. Soc. 2023, 145 (20), 11215–11226.[4] Pearson, R. G. Hard and Soft Acids and Bases. J. Am. Chem. Soc. 1963, 85 (22), 3533–3539.[5] Zhao, S.; Wang, Y.; Hao, Y.; Yin, L.; Kuo, C.-H.; Chen, H.-Y.; Li, L.; Peng, S. Lewis Acid Driving Asymmetric Interfacial Electron Distribution to Stabilize Active Species for Efficient Neutral Water Oxidation. Adv. Mater. 2024, 36 (7), 2308925.[6] Guo, J.; Zheng, Y.; Hu, Z.; Zheng, C.; Mao, J.; Du, K.; Jaroniec, M.; Qiao, S.-Z.; Ling, T. Direct Seawater Electrolysis by Adjusting the Local Reaction Environment of a Catalyst. Nat. Energy 2023, 8 (3), 264–272. Figure 1

Investigating the Redox Dynamics of Rare-Earth Phosphate-Coated Li-Rich Layered Oxides

Li-rich, Mn-rich layered oxides (LMOs) are promising cathode materials for next-generation Li-ion batteries, offering high energy densities (>250 mAhg⁻¹) and an average voltage of ~3.6V. However, LMOs suffer from capacity and voltage decay due to O 2p redox activity, leading to structural degradation, microcracking, lattice destabilization, and O₂ release[1]. This material degradation is driven by both surface and bulk structural transformations, though surface modification has emerged as a key strategy to mitigate these effects by stabilizing surface oxygen bonding and restraining spinel transformation[2].In this work, we investigate the influence of rare earth phosphate (REP) coatings (YPO₄ and CePO₄) on the electronic structure and electrochemical performance of Li₁.₂Mn₀.₅₄Ni₀.₁₃Co₀.₁₃O₂. Using soft X-ray absorption spectroscopy (sXAS), we observe reduced O 2p-TM 3d hole formation, improved reversibility of transition metal redox and dynamic changes in P-O coordination in REP-coated LMOs. Additionally, band alignment at the REP-LMO interface is discovered, correlating with lower observed charge transfer resistance and generally enhanced electrochemical performance during initial and long cycling. This study further demonstrates the importance of electronic compatibility in surface modifications for Li-ion battery cathodes.

Lattice Oxygen Activation through Deep Oxidation of Co4N by Jahn–Teller–Active Dopants for Improved Electrocatalytic Oxygen Evolution

AbstractTriggering the lattice oxygen oxidation mechanism is crucial for improving oxygen evolution reaction (OER) performance, because it could bypass the scaling relation limitation associated with the conventional adsorbate evolution mechanism through the direct formation of oxygen–oxygen bond. High‐valence transition metal sites are favorable for activating the lattice oxygen, but the deep oxidation of pre‐catalysts suffers from a high thermodynamic barrier. Here, taking advantage of the Jahn–Teller (J–T) distortion induced structural instability, we incorporate high‐spin Mn3+ ( ) dopant into Co4N. Mn dopants enable a surface structural transformation from Co4N to CoOOH, and finally to CoO2, as observed by various in situ spectroscopic investigations. Furthermore, the reconstructed surface on Mn‐doped Co4N triggers the lattice oxygen activation, as evidenced experimentally by pH‐dependent OER, tetramethylammonium cation adsorption and online electrochemical mass spectrometry measurements of 18O‐labelled catalysts. In general, this work not only offers the introducing J–T effect approach to regulate the structural transition, but also provides an understanding about the influence of the catalyst's electronic configuration on determining the reaction route, which may inspire the design of more efficient catalysts with activated lattice oxygen.

Lattice Oxygen Activation through Deep Oxidation of Co4N by Jahn-Teller-Active Dopants for Improved Electrocatalytic Oxygen Evolution.

Triggering the lattice oxygen oxidation mechanism is crucial for improving oxygen evolution reaction (OER) performance, because it could bypass the scaling relation limitation associated with the conventional adsorbate evolution mechanism through the direct formation of oxygen-oxygen bond. High-valence transition metal sites are favorable for activating the lattice oxygen, but the deep oxidation of pre-catalysts suffers from a high thermodynamic barrier. Here, taking advantage of the Jahn-Teller (J-T) distortion induced structural instability, we incorporate high-spin Mn3+ ( ) dopant into Co4N. Mn dopants enable a surface structural transformation from Co4N to CoOOH, and finally to CoO2, as observed by various in situ spectroscopic investigations. Furthermore, the reconstructed surface on Mn-doped Co4N triggers the lattice oxygen activation, as evidenced experimentally by pH-dependent OER, tetramethylammonium cation adsorption and online electrochemical mass spectrometry measurements of 18O-labelled catalysts. In general, this work not only offers the introducing J-T effect approach to regulate the structural transition, but also provides an understanding about the influence of the catalyst's electronic configuration on determining the reaction route, which may inspire the design of more efficient catalysts with activated lattice oxygen.

Surface structural transformation and adsorption performance of poly(triazine imide)

Abstract Poly(triazine imide) synthesized by the molten salt route with incorporation of Li+ and Cl‒ ions (PTI/Li+Cl−) was prepared by rational controlling of the washing extent. The surface transformation of the phase-pure PTI framework incurred by thorough washing has been confirmed by a systematic analysis based on x-ray diffraction, Ultraviolet Raman, atomic force microscope, and x-ray photoelectron spectroscopy measurements. Uranyl ion adsorption experiments further support the surface structural transformation from triazine-unit PTI to heptazine-based graphitic carbon nitride. Efforts on the exploration of PTI material application should be mainly directed toward the waterless fields to give good repeatability.

Interface Regulation of Bi3TiNbO9/Rh Photocatalysts by Introducing Ultrathin Heterolayer to Enhance Overall Water Splitting

AbstractAurivillius compounds‐based photocatalysts have attracted extensive interest largely due to their ferroelectric properties and modifiable characteristics arising from the alternate stacking of structural units. However, the interfacial Schottky barrier between such semiconducting compounds as light absorbers and metallic cocatalysts for hydrogen evolution restrains the transfer of photogenerated electrons, resulting in low photocatalytic overall water splitting activity and stability. Here, an anions‐induced surface structure transformation strategy is employed to modulate the interface structure between Bi3TiNbO9 as a typical Aurivillius compound and the cocatalyst Rh by in situ growing ultrathin Bi2MoO6 heterolayer on the surface of Bi3TiNbO9 nanosheets. The introduction of Bi2MoO6 heterolayer lowers the energy barrier near the Bi3TiNbO9 surface, which can facilitate the transfer of the photogenerated electrons from bulk to surface and thus the reduction of Rh cocatalyst for highly active hydrogen production. Compared with the Bi3TiNbO9‐Rh photocatalyst, the proportion of low‐valence metallic Rh0 in Bi2MoO6 heterolayer‐modified Bi3TiNbO9‐Rh (Bi3TiNbO9‐Bi2MoO6‐Rh) is improved by 7.76%, giving rise to a photocatalytic overall water splitting activity enhancement by a factor of 4.74. This strategy emphasizes the importance of interface regulation in promoting the transfer of photogenerated charge carriers in Aurivillius‐type photocatalysts, providing an effective pathway for designing and fabricating high‐performance photocatalysts.

Atomic surface induced by novel green chemical mechanical polishing for aspheric thin-walled crucibles with large diameters

Aspheric thin-walled crucibles with large diameters are widely used for photovoltaics and semiconductor industries, while they have hard-brittle and aspheric characteristics. Nevertheless, these crucibles strictly require atomic surfaces and high material removal rate (MRR). This is a challenge for chemical mechanical polishing (CMP). To break this challenge, a novel green CMP was developed for fused silica, containing ceria, lanthana, hydrogen peroxide, diethylene triaminepentaacetic acid pentasodium, carboxymethyl cellulose sodium and guanidine carbonate. After CMP, surface roughness Ra of 0.115 is obtained at a scanning area of 5 × 5 μm2 measured by atomic force microscopy, and MRR is 563 nm/min. In such an atomic surface, the MRR is the highest compared with those published previously. Transmission electron microscopy confirms that the thickness of damaged layer after CMP is only 2.63 nm. A novel approach of robot floating polishing is proposed for aspheric crucibles with a diameter of 810 mm and a height of 580 mm. Using the developed CMP, surface roughness Sa of 0.178 nm on inner surface of an aspheric thin-walled crucible is achieved under a measurement area of 50 × 50 μm2. X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance reveal that the content of Ce3+ increased from 28.54 to 43.76 %. Through Raman, XPS and Fourier transform infrared spectroscopy, CMP mechanisms are elucidated. Densified damaged layer on the surface of fused silica was induced by the transformation of surface structure from six-membered to four- or three-membered network. Ce-OH and Si-OH were formed between fused silica and ceria abrasives, forming Ce-O-Si by condensation reaction of dehydration. The developed green CMP and approach provide new pathway to polish and manufacture atomic surfaces for aspheric thin-walled crucibles with large diameters.

Cold-welded joint characteristics of gold nanowires via atomistic simulation

This study examines the effects of contact depth, contact orientation, contact angle, and nanowires temperature on the tensile strength of the gold nanowire generated via the cold welding process. There is an increase in the deformation rate with increasing contact depth. From 5 Å to 20 Å, the lattice matching of the nanowires is good, leading to a good weld joint. The greatest ultimate tensile strength (UTS) value is achieved at 5 Å contact depth compared to other depth cases. The UTS value is sensitive to the change in the contact orientation and the contact angle. Compared to other orientations, the (001) vs (001) orientation has the highest rate of orientation matching with the highest FCC structure ratio. Other orientations obtain a much lower UTS value. When the contact angle rises, the UTS value mainly decreases due to the higher level of lattice mismatch. Compared to the other angle cases, the maximum UTS value is attained at 0º contact angle. Moreover, changing in nanowires temperature within the 250 K450 K range do not result in significant variations in the sample's surface morphology, structural transformation, or atomic strain. At 250 K, the sample has the highest UTS value compared to other temperature cases.

Surface and bulk transformations of Ag2CuMnO4 delafossite during the interaction with CO+O2 mixture

Ternary oxide of silver, copper and manganese (Ag2CuMnO4) with delafossite-type structure demonstrates excellent catalytic activity in the reaction of CO oxidation at room temperature and even below. To prepare delafossite-based catalyst the hydrothermal approach using metal nitrates in alkaline solution with an excess of peroxodisulphate was applied. X-ray diffraction pattern of Ag2CuMnO4 particles was successfully simulated taking into an account crystallite shape anisotropy, particle size distribution, the presence of stacking faults, and the lattice expansion along c axis. As-prepared Ag2CuMnO4 sample was characterized by the presence of Ag1+-, Cu2+- and Mn4+-like surface species predominantly. In situ XRD data revealed the thermal stability of delafossite-type structure in catalytic CO+O2 mixture up to 500 °C, while ex situ XPS showed an evident reorganization of Ag2CuMnO4 surface at markedly lower temperatures (200–250 °C). Heating in CO+O2 medium at 150–400 °C also resulted in significant catalytic activation of Ag2CuMnO4 owing to the Mn enrichment and optimization of manganese and copper electronic exchange. The low-temperature activity of Ag2CuMnO4 catalyst in CO+O2 reaction was proposed to be mainly provided by redox transitions with the participation of Cu1+/Cu2+ and Mn3+/Mn4+ couples, while the catalytic role of silver species is considered as auxiliary only. The innovation point of this work is related to the investigation of the dynamics of surface and bulk structure transformations in connection with the catalytic activation of delafossite particles for the low-temperature oxidation.

Enhancing the Stability of 4.6 V LiCoO2 Cathode Material via Gradient Doping.

LiCoO2 (LCO) can deliver ultrahigh discharge capacities as a cathode material for Li-ion batteries when the charging voltage reaches 4.6 V. However, establishing a stable LCO cathode at a high cut-off voltage is a challenge in terms of bulk and surface structural transformation. O2 release, irreversible structural transformation, and interfacial side reactions cause LCO to experience severe capacity degradation and safety problems. To solve these issues, a strategy of gradient Ta doping is proposed to stabilize LCO against structural degradation. Additionally, Ta1-LCO that was tuned with 1.0 mol% Ta doping demonstrated outstanding cycling stability and rate performance. This effect was explained by the strong Ta-O bonds maintaining the lattice oxygen and the increased interlayer spacing enhancing Li+ conductivity. This work offers a practical method for high-energy Li-ion battery cathode material stabilization through the gradient doping of high-valence elements.

Cu2+-induced surface structure transformation on Au nanocubes from a concave to a convex configuration: mechanism and catalytic performance

Surface structures of Au nanocubes were tuned from concave to flat to convex using trace amounts of copper ions, and their corresponding catalytic performances were investigated.

First principle-based rate equation theory for the carbonation kinetics of CaO with CO2 in calcium looping

Calcium oxide (CaO), a CO2 sorbent and key ingredient in the process of making cement, exhibits excellent potential for carbon capture, utilization, and storage (CCUS) applications through calcium looping and integrated carbon capture and utilization, owing to its outstanding kinetic properties and high recyclability. From a fundamental standpoint, it is important to establish a comprehensive and precise carbonation kinetic model for CaO. In the present study, a density functional theory (DFT)-based rate equation was developed to predict the gas–solid reaction kinetics of CaO carbonation with CO2 in calcium looping. The reaction pathway of CaO carbonation, including surface reactions and structural transformations, was obtained by a transition state search of DFT calculations, and the transition state theory was used to calculate the reaction rate constants. The nucleation model was implemented in the mean-field surface reaction model to describe the nucleation of the CaCO3 product at the gas–solid interface. A grain model was developed to couple the surface reaction and CO2 diffusion through the product layer. The developed theory was validated using a wide range of experimental data, and the negative activation energy of CaO carbonation close to equilibrium was accurately predicted. The developed first principle-based rate equation provides a detailed theoretical framework for predicting CaO performance and optimizing the microstructure of Ca-based sorbents.

Manipulating Na occupation and constructing protective film of P2-Na0.67Ni0.33Mn0.67O2 as long-term cycle stability cathode for sodium-ion batteries

P2-Na0.67Ni0.33Mn0.67O2 (NNMO) is promising cathode material for sodium-ion batteries (SIBs) due to its high specific capacity and fast Na+ diffusion rate. Nonetheless, the irreversible P2-O2 phase transformation, Na+/vacancy ordering, and transition metal (TM) dissolution seriously damage its cycling stability and restrict its commercialization process. Herein, Na occupation manipulation and interface stabilization are proposed to strengthen the phase structure of NNMO by synergistic Zn/Ti co-doping and introducing lithium difluorophosp (LiPO2F2) film-forming electrolyte additive. The Zn/Ti co-doping regulates the occupancy ratio of Nae/Naf at Na sites and disorganizes the Na+/vacancy ordering, resulting in a faster Na+ diffusion kinetics and reversible P2-Z phase transition for P2-Na0.67Ni0.28Zn0.05Mn0.62Ti0.05O2 (NNZMTO). Meanwhile, the LiPO2F2 additive can form homogeneous and ultrathin cathode-electrolyte interphase (CEI) on NNZMTO surface, which can stabilize the NNZMTO-electrolyte interface to prevent TM dissolution, surface structure transformation, and micro-crack generation. Combination studies of in situ and ex situ characterizations and theoretical calculations were used to elucidate the storage mechanism of NNZMTO with LiPO2F2 additive. As a result, the NNZMTO displays outstanding capacity retention of 94.44% after 500 cycles at 1C with 0.3 wt% LiPO2F2, excellent rate performance of 92.5 mA h g−1 at 8C with 0.1 wt% LiPO2F2, and remarkable full cell capability. This work highlights the important role of manipulating Na occupation and constructing protective film in the design of layered materials, which provides a promising direction for developing high-performance cathodes for SIBs.

Surface reconstruction and structural transformation of two-dimensional Ni-Fe MOFs for oxygen evolution in seawater media

Surface reconstruction and structural transformation of two-dimensional Ni-Fe MOFs for oxygen evolution in seawater media

Three-dimensional Imaging of Surface Structural Transformations on Electrocatalyst Nanoparticles Using Multi-slice Electron Ptychography.

Three-dimensional Imaging of Surface Structural Transformations on Electrocatalyst Nanoparticles Using Multi-slice Electron Ptychography.

Nanostructured relaxor ferroelectric polymers enable full utilization of nickel-rich cathode at wide-temperature

The sluggish Li+ replenishment and surface structural transformation issues are major bottlenecks that limit the full utilization of Ni-rich cathodes (NLOs) in high-energy rechargeable batteries. Here, a relaxor ferroelectric poly(vinylidene fluoride-trifluoroethylene) (P(VDF + TrFE)) copolymer coated on commercial polypropylene (PP) separator is proposed to tackle these problems for NLO cathodes. It is discovered that the relaxor ferroelectric P(VDF + TrFE) nanostructures with high spontaneous polarization and high dielectric constant at ambient temperature, acts as an ion-selective pump to accelerate the migration of lithium ions and reduce the accumulation of anions at the coating/cathode interface, thereby significantly improving the lithium-ion utilization rate of the cathode, reducing the interfacial side reactions, and stabilizing the cathode structure. As a result, the NCM811/Li cell with the relaxor ferroelectric coating shows extraordinary rate performance and cycling durability at a wide-temperature range of −40 ∼ 55 ℃. This work sheds light on the possibility to improve the electrochemical performance of other intercalated electrode materials by engineering ferroelectric interlayers.

In Situ EC-AFM Study of the Initial Stages of Cathodic Corrosion of Pt(111) and Polycrystalline Pt in Acid Solution.

An atomic scale understanding of the surface degradation mechanism during cathodic corrosion of a platinum electrode is still lacking. Here, we present results of surface structural changes observed during cathodic polarization of a polycrystalline Pt electrode and single crystalline Pt(111) in acid electrolytes in the absence and presence of cations (Na+) by in situ electrochemical atomic force microscopy (EC-AFM) imaging. The electrolyte cation is proved to be a prerequisite to trigger cathodic etching of the polycrystalline Pt surface. Further examination of the evolution of electrochemical signals and distinct surface structural transformations of an atomically defined Pt(111) single-crystal electrode during cathodic corrosion reveals clearly that the roughening process commences at the under-coordinated sites of the Pt(111) surface. The created triangular-shape pattern, actually a 100-oriented pit in a 111-terrace, grows primarily laterally in the initial regime, while prolonged cathodic corrosion leads to the existing etching pits growing in depth until ultimately they coalesce with each other, generating a highly roughened surface.

Surface morphologies, chemical compositions and luminescent properties of ZnO thin films flattened by ion milling procedure

The luminescent properties of ZnO thin films have been developed to form a nanometric light source for an electron beam-excitation assisted optical microscope. Spatial uniformity of the electron-induced luminescence is important when using this luminescent ZnO film-based light source. In this work, ZnO thin film surfaces have been flattened using broad ion beam polishing with Ar+ ions at a grazing angle of incidence. Atomic force microscopy and electron microscopy confirmed transformation of needle-like ZnO surface structures into scale-like morphologies during ion irradiation. The root-square-mean roughness, corresponding to the height distribution decreased dramatically from 32 to 8 nm after 5 keV Ar+ ion irradiation for 60 min. X-ray photoelectron spectroscopy and Monte Carlo simulations revealed changes in the chemical state and lattice defects during ion irradiation. Positive shifts in the binding energies of the Zn 2p and O 1s spectra suggested preferential Zn atom sputtering and chemical composition change from oxygen-deficient to stoichiometric ZnO. The ultraviolet region luminescent intensity improved after ion irradiation for 60 min. Valence band electron redistribution was considered to influence the ZnO thin film’s luminescent properties. We propose that homogeneous luminescence over a specified area with flat surfaces can be accomplished by repeated short-duration ion irradiation.

Topotactic Transformation of Surface Structure Enabling Direct Regeneration of Spent Lithium-Ion Battery Cathodes.

Recycling spent lithium-ion batteries (LIBs) has become an urgent task to address the issues of resource shortage and potential environmental pollution. However, direct recycling of the spent LiNi0.5Co0.2Mn0.3O2 (NCM523) cathode is challenging because the strong electrostatic repulsion from a transition metal octahedron in the lithium layer provided by the rock salt/spinel phase that is formed on the surface of the cycled cathode severely disrupts Li+ transport, which restrains lithium replenishment during regeneration, resulting in the regenerated cathode with inferior capacity and cycling performance. Here, we propose the topotactic transformation of the stable rock salt/spinel phase into Ni0.5Co0.2Mn0.3(OH)2 and then back to the NCM523 cathode. As a result, a topotactic relithiation reaction with low migration barriers occurs with facile Li+ transport in a channel (from one octahedral site to another, passing through a tetrahedral intermediate) with weakened electrostatic repulsion, which greatly improves lithium replenishment during regeneration. In addition, the proposed method can be extended to repair spent NCM523 black mass, spent LiNi0.6Co0.2Mn0.2O2, and spent LiCoO2 cathodes, whose electrochemical performance after regeneration is comparable to that of the commercial pristine cathodes. This work demonstrates a fast topotactic relithiation process during regeneration by modifying Li+ transport channels, providing a unique perspective on the regeneration of spent LIB cathodes.

Effect of ‘in-plane’ contraction on the (0 0 1) surface of the model perovskite SrTiO3

Reflection high-energy electron diffraction in the temperature range 8–300 K revealed several surface structural transformations accompanied with a strong ‘in-plane’ lattice parameter growth on the (001) surface of the model perovskite SrTiO3 single crystal. Rather may occurring at sharp temperatures, these transformations are observed to extend over temperature intervals separated from each other by temperature zones where no structural changes occur and the crystal lattice is relaxed. In such relaxed zones the equilibrium lattice parameters were identified and analyzed as a function of penetration depth. A strong ‘in-plane’ contraction of the crystal lattice in the first surface layers was experimentally revealed at low temperatures and theoretically confirmed by hybrid DFT calculations. Along with such an ‘in-plane’ contraction of the (001) surface at low temperatures we have found that the thermal expansion on the surface is much higher than in the bulk which results in approaching of the surface ‘in-plane’ and the bulk lattice parameters at room temperature. This contradicts the general assumption that surface ‘in-plane’ and bulk lattice parameters are identical.