Low-temperature Orthorhombic Structure Research Articles

Pre-existing orthorhombic embryos-induced hexagonal–orthorhombic martensitic transformation in MnNiSi1–x (CoNiGe) x alloy

The thermal–elastic martensitic transformation from high-temperature Ni2In-type hexagonal structure to low-temperature TiNiSi-type orthorhombic structure has been widely studied in MnMX (M = Ni or Co, and X = Ge or Si) alloys. However, the answer to how the orthorhombic martensite nucleates and grows within the hexagonal parent is still unclear. In this work, the hexagonal–orthorhombic martensitic transformation in a Co and Ge co-substituted MnNiSi is investigated. One can find some orthorhombic laths embedded in the hexagonal parent at a temperature above the martensitic transformation start temperature (M s). With the the sample cooing to M s, the laths turn broader, indicating that the martensitic transformation starts from these pre-existing orthorhombic laths. Microstructure observation suggests that these pre-existing orthorhombic laths do not originate from the hexagonal–orthorhombic martensitic transformation because of the difference between atomic occupations of doping elements in the hexagonal parent and those in the pre-existing orthorhombic laths. The phenomenological crystallographic theory and experimental investigations prove that the pre-existing orthorhombic lath and generated orthorhombic martensite have the same crystallography relationship to the hexagonal parent. Therefore, the orthorhombic martensite can take these pre-existing laths as embryos and grow up. This work implies that the martensitic transformation in MnNiSi1−x (CoNiGe) x alloy is initiated by orthorhombic embryos.

Structural transitions and magnetocaloric properties of low-cost MnNiSi-based intermetallics

A series of Mn1-xNi1-xFe2xSi0.95Al0.05 MM’X-type compounds (with x = 0.28, 0.3, 0.32 and 0.35) were investigated for their potential as magnetocaloric materials. Structural and magnetic properties were studied by magnetometry, microscopy and X-ray diffraction. Double substitution of Fe in Mn and Ni sites allowed to tune martensitic transition temperatures between low temperature orthorhombic and high temperature hexagonal structures from 373 K in x = 0.28–183 K in x = 0.35 during cooling. Transition temperatures occur around room temperature for x = 0.30 (300 K for cooling transformation) and 0.32 (270 for heating transformation). Isothermal entropy changes of −8, −19 and −26 J/kg.K were calculated for field changes of μ0H = 0–2, 0–5 and 0–7 T for x = 0.30. The values are comparable to those reported for MnNi(SiAl)-based compounds with single site substitutions (Mn and Ni) by Fe. Further analyses show that high magnetic fields are necessary to induce the magnetostructural transition in all studied compounds, which can be attributed to the presence of secondary phases and/or disorder at a local level.

A brief review of the physical properties of charge density wave superconductor LaPt2Si2

The study of materials with multiple phases, such as superconductivity (SC) coexisting with charge density wave (CDW) or spin density wave (SDW) instability, attracts considerable interest from the condensed matter research community. The CDW superconductors started drawing in heaps of attention soon after the discovery of CDW instability in high-T c cuprates, where understanding the underlying superconducting mechanism of the latter may turn out to be path-breaking for the discovery of room temperature SC. Understanding the pairing mechanism of high-T c superconductors necessitates less complex systems and this makes searching for CDW superconductors all the more important. Such systems avoid the additional complexity in contrast to the well-sought after Fe-based superconductors, which show more competing orders like SDW, nematicity and SC. RPt2Si2 (R = La, Pr, Eu) is a recently discovered series of materials, members of which crystallizes in CaBe2Ge2 type structure which has a close resemblance to the ThCr2Si2 type structure commonly found in pnictide-122 superconductors. This review is focused on LaPt2Si2, which undergoes a structural transition from high-temperature tetragonal to low-temperature orthorhombic structure, accompanied by a CDW transition around 112 K, which is then followed by a superconducting transition below 1.8 K. We discuss the physical properties of single crystal and polycrystalline LaPt2Si2 samples. Additionally, we present the results of transport and ac susceptibility measurements under external hydrostatic pressure to map out the temperature-pressure phase diagram of LaPt2Si2.

Nematic state of the FeSe superconductor

We study the crystal structure of the tetragonal iron selenide FeSe and its nematic phase transition to the low-temperature orthorhombic structure using synchrotron x-ray and neutron scattering analyzed in both real and reciprocal space. We show that in the local structure the orthorhombic distortion associated with the electronically driven nematic order is more pronounced at short length scales. It also survives up to temperatures above 90 K where reciprocal-space analysis suggests tetragonal symmetry. Additionally, the real-space pair distribution function analysis of the synchrotron x-ray diffraction data reveals a tiny broadening of the peaks corresponding to the nearest Fe-Fe, nearest Fe-Se, and the next-nearest Fe-Se bond distances as well as the tetrahedral torsion angles at a short length scale of 20 angstr\"om. This broadening appears below 20 K and is attributed to a pseudogap. However, we did not observe any further reduction in local symmetry below orthorhombic down to 3 K. Our results suggest that the superconducting gap anisotropy in FeSe is not associated with any symmetry-lowering short-range structural correlations.

Doping site induced alteration of incommensurate antiferromagnetic ordering in Fe-doped MnNiGe alloys

The magnetic structure of Fe-doped MnNiGe alloys of nominal compositions ${\mathrm{MnNi}}_{0.75}{\mathrm{Fe}}_{0.25}\mathrm{Ge}$ and ${\mathrm{Mn}}_{0.85}{\mathrm{Fe}}_{0.15}\mathrm{NiGe}$ has been explored through detailed neutron powder diffraction (NPD) study in ambient and high-pressure (6 kbar) conditions. Both these alloys crystallize with ${\mathrm{Ni}}_{2}\mathrm{In}$-type hexagonal structure (with $P{6}_{3}/mmc$ space group) at room temperature and undergo a martensitic transition to low-temperature TiNiSi-type orthorhombic structure (with space group $Pnma$). Both the alloys show incommensurate antiferromagnetic ordering at the low-temperature martensitic phase. However, the modulation of the magnetic structures depends strongly on the doping site of Fe. The incommensurate propagation vector $\mathbf{k}$ for ${\mathrm{MnNi}}_{0.75}{\mathrm{Fe}}_{0.25}\mathrm{Ge}$ and ${\mathrm{Mn}}_{0.85}{\mathrm{Fe}}_{0.15}\mathrm{NiGe}$ alloys are found to be (0.1790(1),0,0) and (0.1543(3),0,0), respectively at 1.5 K, and these remain almost unchanged with increasing sample temperature under ambient conditions. The application of the external pressure results in a significant effect on both martensitic transition temperature and low-temperature magnetic structure. The propagation vectors $\mathbf{k}$ for both the alloys show a monotonic decrease with increasing sample temperature in the presence of external pressure. Interestingly, no sign of the magnetically ordered hexagonal austenite phase was observed in any alloys down to the lowest measurement temperature.

Hidden competing phase revealed by first-principles calculations of phonon instability in the nearly optimally doped cuprate La1.875Sr0.125CuO4

The representative cuprate, La$_{2-x}$M$_x$CuO$_4$, with M = Sr and $x = 1/8$ is studied via first-principles calculations in the high-temperature tetragonal (HTT), low-temperature orthorhombic (LTO), and low-temperature less-orthorhombic (LTLO) structures. By suppressing the magnetism and superconductivity, the LTLO phase, which has rarely been observed in La$_{2-x}$Sr$_x$CuO$_4$, is found to be the ground state, where the structural phase transitions, HTT$\rightarrow$LTO$\rightarrow$LTLO, can be understood via phonon instability. While the La-O composition is identified to be responsible for the phonon softening, the superconducting CuO$_2$ layer is dynamically stable. The LTLO phase, which can exhibit a $\sim$20 meV splitting in the density of states, is proposed to have an intimate relationship with the observed pseudogap and the charge density wave giving the stripe. We argue that at low temperatures, the superconducting LTO La$_{1.875}$Sr$_{0.125}$CuO$_4$ competes with the phonon-preferred LTLO phase by spontaneously forming the Cooper pairs, resulting in suppressing the stripe. Therefore, the revealed LTLO phase is indispensable for understanding La$_{2-x}$Sr$_x$CuO$_4$.

Self-trapped holes (small polarons) in ferroelectric KH2PO4 crystals

Density functional theory is used to establish the ground-state structure of the self-trapped hole (STH) in KH2PO4 crystals. The STHs in this nonlinear optical material are free small polarons, a fundamental intrinsic point defect. They are produced with ionizing radiation in the low-temperature orthorhombic structure of KH2PO4 and are only stable (i.e. long-lived) below approximately 70 K. A large 129-atom cluster, K19H40P14O56, is constructed to model the STH. The ωB97XD functional with the 6−31+G* basis set is used and geometry optimization is performed. Our results show that two of the oxygen ions in a PO4 unit relax toward each other and equally share the hole. These two oxygen ions do not initially have close hydrogen neighbors. This equal sharing of the hole is related to the presence of isolated, slightly distorted, PO4 units and is significantly different from the small-polaron behavior often observed in other oxide crystals where the hole is localized on only one oxygen ion. The computational results provide a detailed description of the lattice relaxation occurring during formation of the STH. Characteristic spectral features of this defect are a larger hyperfine interaction with one 31P nucleus and equal, but smaller, hyperfine interactions with two 1H nuclei. The computed values for these isotropic and anisotropic hyperfine coupling constants are in excellent agreement with results obtained from electron paramagnetic resonance experiments.

Magneto-structural correlation in Co0.8Cu0.2Cr2O4 cubic spinel

Neutron and X-ray diffraction, magnetic susceptibility, and specific heat measurements have been used to investigate the magneto-structural phase transitions in 20% Cu substituted multiferroic CoCr2O4 spinel. The Jahn-Teller active Cu2+ ion in the tetrahedral A-site of the spinel configuration induces the Jahn-Teller distortion slightly above the Néel temperature. In this compound, we observe a Jahn-Teller distortion of the crystal structure at 90 K. It was further observed that the high temperature cubic (Fd3‾m) structure coexists with the low temperature orthorhombic (Fddd) structure till the lowest temperature of measurement.

Thermoelectric and structural correlations in (Sr1−x−yCaxNdy)TiO3 perovskites

Structural and thermoelectric properties are reported for a specially designed class of $A$-site substituted perovskite titanates, $(\mathrm{S}{\mathrm{r}}_{1\ensuremath{-}x\ensuremath{-}y}\mathrm{C}{\mathrm{a}}_{x}\mathrm{N}{\mathrm{d}}_{y})\mathrm{Ti}{\mathrm{O}}_{3}$. Two series synthesized with various $A$-site Sr-rich or Ca-rich (Sr-poor) concentrations were investigated using high-resolution neutron powder diffraction as a function of temperature and Nd doping. Each series was designed to have a nominally constant tolerance factor at room temperature. We determine the room temperature structures as tetragonal $I4/mcm$ and orthorhombic $Pbnm$ for the Sr-rich and Ca-rich series, respectively. Three low-temperature orthorhombic structures, $Pbnm, Ibmm$, and $Pbcm$ were also observed for the Sr-rich series, whereas the symmetry of the Ca-rich series remains unchanged throughout the full measured temperature range. Thermoelectric properties of $(\mathrm{S}{\mathrm{r}}_{1\ensuremath{-}x\ensuremath{-}y}\mathrm{C}{\mathrm{a}}_{x}\mathrm{N}{\mathrm{d}}_{y})\mathrm{Ti}{\mathrm{O}}_{3}$ were investigated and correlated with the structural variables. We succeeded in achieving a relatively high figure of merit $ZT=0.07$ at $\ensuremath{\sim}400\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ in the Sr-rich $\mathrm{S}{\mathrm{r}}_{0.76}\mathrm{C}{\mathrm{a}}_{0.16}\mathrm{N}{\mathrm{d}}_{0.08}\mathrm{Ti}{\mathrm{O}}_{3}$ composition which is comparable to that of the best $n$-type TE $\mathrm{SrT}{\mathrm{i}}_{0.80}\mathrm{N}{\mathrm{b}}_{0.20}{\mathrm{O}}_{3}$ oxide material reported to date. For a fixed tolerance factor, the Nd doping enhances the carrier density and effective mass at the expense of the Seebeck coefficient. Thermal conductivity greatly reduces upon Nd doping in the Ca-rich series. With an enhanced Seebeck coefficient at elevated temperatures and reduced thermal conductivity, we predict that $\mathrm{S}{\mathrm{r}}_{0.76}\mathrm{C}{\mathrm{a}}_{0.16}\mathrm{N}{\mathrm{d}}_{0.08}\mathrm{Ti}{\mathrm{O}}_{3}$ and similar compositions have the potential to become some of the best materials in their class of thermoelectric oxides.

Polarized Light Microscopy Study on the Reentrant Phase Transition in a (Ba1 – xKx)Fe2As2 Single Crystal with x = 0.24

A sequence of structural/magnetic transitions on cooling is reported in the literature for hole-doped iron-based superconductor (Ba1 − xKx)Fe2As2 with x = 0.24. By using polarized light microscopy, we directly observe the formation of orthorhombic domains in (Ba1 − xKx)Fe2As2 (x = 0.24) single crystal below a temperature of simultaneous structural/magnetic transition TN ~ 80 K. The structural domains vanish below ~30 K, but reappear below T = 15 K. Our results are consistent with reentrance transformation sequence from high-temperature tetragonal (HTT) to low temperature orthorhombic (LTO1) structure at TN ~ 80 K, LTO1 to low temperature tetragonal (LTT) structure at Tc ~ 25 K, and LTT to low temperature orthorhombic (LTO2) structure at T ~ 15 K.

MoTe_{2}: A Type-II Weyl Topological Metal.

Based on the abinitio calculations, we show that MoTe_{2}, in its low-temperature orthorhombic structure characterized by an x-ray diffraction study at 100K, realizes 4 type-II Weyl points between the Nth and (N+1)th bands, where N is the total number of valence electrons per unit cell. Other WPs and nodal lines between different other bands also appear close to the Fermi level due to a complex topological band structure. We predict a series of strain-driven topological phase transitions in this compound, opening a wide range of possible experimental realizations of different topological semimetal phases. Crucially, with no strain, the number of observable surface Fermi arcs in this material is 2-the smallest number of arcs consistent with time-reversal symmetry.

Magnetostructural transition and magnetocaloric effect in MnCoGe–NiCoGe system

Magnetostructural transition and magnetocaloric effect were investigated for MnCoGe–NiCoGe system. Chemically alloying NiCoGe with the parent compound MnCoGe stabilizes the high-temperature hexagonal structure relative to the low-temperature orthorhombic structure and keeps their Curie temperatures nearly constants, establishing a Curie temperature window in which the magnetic and structural transitions coincide. As a result, magnetostructural transitions occur between ferromagnetic and paramagnetic states with sharp jumps in magnetization around room temperature. Large magnetocaloric effects with tunable phase transition temperatures were observed in the MnCoGe–NiCoGe system.

Ti substitution for Mn in MnCoGe – The magnetism of Mn0.9Ti0.1CoGe

Abstract Bulk magnetization measurements (5–320 K; 0–8 T) reveal that below room temperature Mn0.9Ti0.1CoGe exhibits two magnetic phase transitions at ∼178 K and ∼280 K. Neutron diffraction measurements (3–350 K) confirm that the transition at ∼178 K is due to the structural change from the low-temperature orthorhombic TiNiSi-type structure (space group Pnma) to the higher temperature hexagonal Ni2In-type structure (space group P63/mmc), while the transition at ∼280 K originates from the transition from ferromagnetism to paramagnetism. The magnetocaloric behaviour of Mn0.9Ti0.1CoGe around Tstr ∼ 178 K and TC ∼ 280 K as determined via the magnetic field and temperature dependences of DC magnetisation are given by the maximum values of the magnetic entropy changes −Δ S M max = 6.6 J kg−1 K−1 around Tstr ∼ 178 K, and −Δ S M max = 4.2 J kg−1 K−1 around TC ∼ 280 K for a magnetic field change of ΔB = 0–8 T. Both structural entropy – due to the unit cell expansion of ∼4.04% – and magnetic entropy – due to an increase in the magnetic moment of ∼31% – are found to contribute significantly to the total entropy change around Tstr. Critical analysis of the transition around TC ∼ 280 K leads to exponents similar to values derived from a mean field theory, consistent with long-range ferromagnetic interactions. It was found that the field dependence of −Δ S M max can be expressed as −Δ S M max ∝ Bn with n = 1 for the structural transition around Tstr and n = 2/3 for the ferromagnetic transition around TC, thereby confirming the second order nature of this latter transition.

On the crystal structure and magnetic properties of the Mn0.94Ti0.06CoGe alloy

Structural and magnetic properties of Mn0.94Ti0.06CoGe have been studied by a combination of bulk magnetisation and neutron diffraction measurements over the temperature range of 5 K–350 K. The crystal structural transition occurs at Tstr (∼235 K) with a change in symmetry from the low temperature orthorhombic TiNiSi-type structure (space group Pnma) to the high temperature hexagonal Ni2In-type structure (space group P63/mmc) and the magnetic phase transition takes place around TC = 270 K. It is found that the structural transition around Tstr is incomplete and there is a co-existence of the orthorhombic and hexagonal structures between Tstr and TC (∼270 K). These results are discussed in connection with the magnetic and magnetocaloric behaviours in Mn0.94Ti0.06CoGe.

The magnetocaloric effect and critical behaviour of the Mn0.94Ti0.06CoGe alloy

Structural, magnetic and magnetocaloric properties of the Mn0.94Ti0.06CoGe alloy have been investigated using x-ray diffraction, DC magnetization and neutron diffraction measurements. Two phase transitions have been detected, at Tstr = 235 K and TC = 270 K. A giant magnetocaloric effect has been obtained at around Tstr associated with a structural phase transition from the low temperature orthorhombic TiNiSi-type structure to the high temperature hexagonal Ni2In-type structure, which is confirmed by neutron study. In the vicinity of the structural transition, at Tstr, the magnetic entropy change, −ΔSM reached a maximum value of 14.8 J kg−1 K−1 under a magnetic field of 5 T, which is much higher than that previously reported for the parent compound MnCoGe. To investigate the nature of the magnetic phase transition around TC = 270 K from the ferromagnetic to the paramagnetic state, we performed a detailed critical exponent study. The critical components γ, β and δ determined using the Kouvel–Fisher method, the modified Arrott plot and the critical isotherm analysis agree well. The values deduced for the critical exponents are close to the theoretical prediction from the mean-field model, indicating that the magnetic interactions are long range. On the basis of these critical exponents, the magnetization, field and temperature data around TC collapse onto two curves obeying the single scaling equation M(H,ε) = εβf ± (H/εβ+γ).

Analysis of structural and electronic properties of Pr2NiO4 through first-principles calculations

The structural and electronic properties of bulk Pr2NiO4+δ (δ = 0 and 0.031) were analyzed using first-principles calculations based on the density functional theory (DFT) for application to electrode materials in solid-oxide fuel cells (SOFCs). Two structures of Pr2NiO4 were analyzed: one in space group I4/mmm associated with the high temperature tetragonal (HTT) structure, and the other in Bmab with the low temperature orthorhombic (LTO) structure. The main difference between the two structures is the pronounced tilting of the nickelate octahedra found in the Bmab structure. Here, we will show that the difference in the electronic properties between the two structures, i.e. half-metallic for the I4/mmm structure and metallic for the Bmab structure, is attributed to the tilting of the nickelate octahedra. Furthermore, we found that the presence of interstitial O atoms at the Pr2O2 bilayers is responsible for the tilting of the octahedra and thus is a dominant factor in the transition from the I4/mmm structure to the Bmab structure. These results would be of great significance to materials design related to the enhancement of O diffusivity in this material.

Structures and Phase Transitions of CePd3+xGa8-x: New Variants of the BaHg11 Structure Type

New distorted variants of the cubic BaHg11 structure type have been synthesized in Ga flux. Multiple phases of CePd3+xGa8-x, which include an orthorhombic Pmmn structure (x = 3.21(2)), a rhombohedral R3m structure (x = 3.13(4)), and a cubic Fm3m superstructure (x = 2.69(6)), form preferentially depending on reaction cooling rate and isolation temperature. Differential thermal analysis and in situ temperature-dependent powder X-ray diffraction patterns show a reversible phase transition at approximately 640 °C between the low temperature orthorhombic and rhombohedral structures and the high temperature cubic superstructure. Single crystal X-ray diffraction experiments indicate that the general structure of BaHg11, including the intersecting planes of a kagomé-type arrangement of Ce atoms, is only slightly distorted in the low temperature phases. A combination of Kondo, crystal electric field, and magnetic frustration effects may be present, resulting in low temperature anomalies in magnetic susceptibility, electrical resistivity, and heat capacity measurements. In addition to CePd3+xGa8-x, the rare earth analogues REPd3+xGa8-x, RE = La, Nd, Sm, Tm, and Yb, were successfully synthesized and also crystallize in one of the lower symmetry space groups.

139La NMR investigation in underdoped La1.93Sr0.07CuO4

We report ${}^{139}$La and ${}^{63}$Cu nuclear magnetic and quadrupole resonance (NMR/NQR) studies in an underdoped La${}_{1.93}$Sr${}_{0.07}$CuO${}_{4}$ single crystal, focusing on the ${}^{139}$La NMR in the normal state. We demonstrate that the local structural distortions in the low-temperature orthorhombic structure cause the tilting of the direction of the electric field gradient (EFG) at the nuclei from the $c$ axis, resulting in two NMR central transition spectra at both the ${}^{139}$La and ${}^{63}$Cu nuclei in an external field. Taking into account the tilt angle of the EFG, the temperature dependence of the ${}^{139}$La spectra allowed us to determine the ${}^{139}$La Knight shift and the structural order parameter. The angle and temperature dependence of the ${}^{139}$La spectrum is in perfect agreement with the macroscopic average structure and proves a displacive transition. The ${}^{139}$La nuclear spin-lattice relaxation rates, ${T}_{1}^{\ensuremath{-}1}$, suggest that La${}_{1.93}$Sr${}_{0.07}$CuO${}_{4}$ undergoes a gradual change to a temperature-independent paramagnetic regime in the high-temperature region. Both the spectra and ${T}_{1}^{\ensuremath{-}1}$ of the ${}^{139}$La as a function of temperature reveal a sharp anomaly around ${T}_{S}=387(1)$ K, implying a first-order-like structural transition, and a dramatic change below $\ensuremath{\sim}70$ K arising from collective glassy spin freezing.

Structural and magnetic phase transitions of the orthovanadatesRVO3(R=Dy, Ho, Er) as seen via neutron diffraction

The structural and magnetic phase behavior of $R$VO${}_{3}$ with $R$$=$v Dy, Ho, and Er was studied by single-crystal neutron diffraction. Upon cooling, all three compounds show structural transitions from orthorhombic (space group Pbnm) to monoclinic ($p$2${}_{1}$/$b$) symmetry due to the onset of orbital order at $T$$=$ 188--200 K, followed by N\'eel transitions at $T$$=$ 110--113 K due to the onset of antiferromagnetic ($C$-type) order of the vanadium moments. Upon further cooling, additional structural phase transitions occur for DyVO${}_{3}$ and ErVO${}_{3}$ at 60 and 56 K, respectively, where the monoclinic structure changes to an orthorhombic structure with the space group Pbnm, and the magnetic order of the V sublattice changes to a $G$-type structure. These transition temperatures are reduced compared to the ones previously observed for nonmagnetic $R$${}^{3+}$ ions due to exchange interactions between the ${\mathrm{V}}^{3+}$ and ${R}^{3+}$ ions. For ErVO${}_{3}$, $R$-$R$ exchange interactions drive a transition to collinear magnetic order at $T$$=$ 2.5 K. For HoVO${}_{3}$, the onset of noncollinear, weakly ferromagnetic order of the Ho moments nearly coincides with the structural phase transition from the monoclinic to the low-temperature orthorhombic structure. This transition is characterized by an extended hysteresis between 24 and 36 K. The Dy moments in DyVO${}_{3}$ also exhibit noncollinear, weakly ferromagnetic order upon cooling below 13 K. With increasing temperature, the monoclinic structure of DyVO${}_{3}$ reappears in the temperature range between 13 and 23 K. This reentrant structural transition is associated with a rearrangement of the Dy moments. A group theoretical analysis showed that the observed magnetic states of the ${R}^{3+}$ ions are compatible with the lattice structure. The results are discussed in the light of recent data on the magnetic field dependence of the lattice structure and magnetization of DyVO${}_{3}$ and HoVO${}_{3}$.

Neutron scattering study of the interplay between structure and magnetism inBa(Fe1−xCox)2As2

Single-crystal neutron diffraction is used to investigate the magnetic and structural phase diagrams of the electron-doped superconductor $\text{Ba}{({\text{Fe}}_{1\ensuremath{-}x}{\text{Co}}_{x})}_{2}{\text{As}}_{2}$. Heat-capacity and resistivity measurements have demonstrated that Co doping this system splits the combined antiferromagnetic and structural transition present in ${\text{BaFe}}_{2}{\text{As}}_{2}$ into two distinct transitions. For $x=0.025$, we find that the upper transition is between the high-temperature tetragonal and low-temperature orthorhombic structures with $({T}_{\text{TO}}=99\ifmmode\pm\else\textpm\fi{}0.5\text{ }\text{K})$ and the antiferromagnetic transition occurs at ${T}_{\text{AF}}=93\ifmmode\pm\else\textpm\fi{}0.5\text{ }\text{K}$. We find that doping rapidly suppresses the antiferromagnetism, with antiferromagnetic order disappearing at $x\ensuremath{\approx}0.055$. However, there is a region of coexistence of antiferromagnetism and signatures of superconductivity obtained from thermodynamic and transport properties. For all the compositions studied, we find two anomalies in the temperature dependence of the structural Bragg peaks from both neutron scattering and x-ray diffraction at the same temperatures where anomalies in the heat capacity and resistivity have been previously identified. Thus for $x=0.025$, where we have shown that the lower anomaly occurs at ${T}_{\text{AF}}$, we infer that there is strong coupling between the antiferromagnetism and the crystal lattice which may persist to larger $x$.