Lattice collapse and the magnetic phase diagram of Sr 1−x Ca x Co 2 P 2
aa r X i v : . [ c ond - m a t . s t r- e l ] S e p Lattice collapse and the magnetic phase diagram of Sr − x Ca x Co P Shuang Jia , A. J. Williams , P. W. Stephens , R. J. Cava Department of Chemistry, Princeton University, Princeton, NJ 08544, USA Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY 11794, USA
We report that the 122 type Sr − x Ca x Co P solid solution undergoes an anomalous structuraltransition from the uncollapsed to the collapsed ThCr Si structure at a distinct onset compositionnear x = 0 .
5. Correlated with the structural changes, the electronic system evolves from a nearlyferromagnetic Fermi liquid to an antiferromagnetic metal, through a complex crossover regime. Thestructural collapse, driven by P-P bonding across the (Sr,Ca) layers, is much more pronounced inthis system than it is in the analogous Fe-based system, indicating a strong sensitivity of structureto total electron count in the transition metal pnictide 122 family.
PACS numbers: 61.50.Ks, 74.25.Jb, 75.30.-m
I. INTRODUCTION
The layered ThCr Si
122 structure type is commonlyobserved for AT X compounds based on large (A), tran-sition metal (T), and metalloid (X) atoms. In this 122structure, the T X layers, made from edge-sharing TX tetrahedra, display a wide range of properties, from mag-netic ordering to superconductivity. Early theoretical in-vestigation of this structure type argued for the criticalimportance of the shape of the TX tetrahedra and X-Xbonding across the A layers in determining the electronicstates at the Fermi level. This has again come to thefore in recent research into the structure-property rela-tionships in the iron pnictide superconductors . Onestructural feature of particular interest in the 122 tran-sition metal pnictides is the so-called lattice collapse:some AT P and AT As compounds manifest signifi-cantly smaller ratios of stacking to in-plane lattice pa-rameters ( c/a ) than are expected from simple atomic sizeconsiderations . These are called collapsed tetragonal(cT) cells, and occur because X-X bonding between T X layers pulls the layers closer and induces a relaxation ofthe in-plane lattice dimension. The materials with un-collapsed (ucT) cells are more normal representatives ofthe structure type, with no X-X bonding present. Insome cases the lattice collapse causes a significant differ-ence in Fermi surface topology . CaFe As undergoesa first-order transition from an ucT to a cT phase underpressure. Here we describe the correlations between structureand properties for the Sr − x Ca x Co P solid solution.The pure Sr and pure Ca end members show a highlyanomalous difference in c/a , indicative of a transitionfrom ucT to cT phases . Unlike what is expectedfrom simple Vegard’s law behavior, here we show thatthe collapse onsets suddenly in the middle of the solidsolution series, even though it is driven by smoothlyincreasing P-P bonding across the (Sr,Ca) layer. Themagnetic properties of the end member compounds aredistinct: SrCo P is a nearly ferromagnetic metal withstrongly temperature dependent magnetic susceptibility,whereas CaCo P displays an antiferromagnetic (AFM) transition in which the cobalt moments are orderedferromagnetically within the basal ab plane but anti-ferromagnetically along the c axis (A-type AFM). Employing diffraction, thermodynamic, magnetic, andtransport measurements, we show that the changes inthe magnetic properties of the solid solution correlatewith the structural anomalies. The ground states varyfrom nearly ferromagnetic Fermi liquid (NFFL) to AFM,then to FM-like, and finally back to AFM. The corre-lations between the structure and magnetic propertiesindicate that the electronic structure at the Fermi levelfor Sr − x Ca x Co P is exceptionally strongly dependenton variations in P-P bonding when compared to othercompounds in the same structural family. II. EXPERIMENTAL METHODS
Polycrystalline samples were prepared from elementalP, Sr and Ca, and CoP powder . All the samples werecharacterized by laboratory x-ray diffraction (XRD) withCu Kα radiation (D8 Focus, Bruker). In order to char-acterize the shapes of the CoP tetrahedra and the P-Pdistances between layers, selected samples, with nominal x equaling 0, 0.2, 0.4, 0.6, 0.7, 0.8, 0.9 and 1.0 were mea-sured by synchrotron powder x-ray diffraction (SXRD)at room temperature at beam line X16C at the NationalSynchrotron Light Source at Brookhaven National Lab-oratory. Structure analysis was performed by using theprogram GSAS with EXPGUI . The refined Ca con-centrations x were 1% - 5% larger than the nominal x .Therefore, the x values for all samples with the excep-tion of x = 0 and 1 were linearly calibrated to the truerefined values . All physical property characterizationwas performed on a Quantum Design physical propertymeasurement system (PPMS). III. RESULTS
The lattice parameters of the Sr − x Ca x Co P seriesvary dramatically with composition (Fig. 1). The c axis, Sr Ca x Co P Sr Ca x Fe P CoFe c ( ¯ ) x CoFe a ( ¯ ) FIG. 1: (Color online) The lattice parameters forSr − x Ca x Co P and Sr − x Ca x Fe P cTucT x C o - C o B ond ( ¯ ) C o - P - C o ang l e ( o ) Sr Ca x Co P P - P bond ( ¯ ) Co-P-CoP-P
FIG. 2: (Color online) P-P and Co-Co bond length, as wellas the Co-P-Co tetrahedral angle for Sr − x Ca x Co P . Inset:the structure of CaCo P unit sell. a measure of the unit cell perpendicular to the T X layers, decreases monotonically, but nonlinearly, with x .The a axis, however, a measure of the T X in-plane di-mensions, changes in a highly non-Vegard’s-law, S -shapemanner with composition, showing minimum and maxi-mum values at x ∼ . . (a) Sr Ca x Co P H = 10 kOe M / H ( e m u / m o l C o ) (b) H = 1 kOe M / H ( e m u / m o l C o ) H = 1 kOe (c)
T (K)1.00.800.95 M / H ( e m u / m o l C o ) H=1 kOex = 0.85 M / H ( e m u / m o l C o ) T (K)
FIG. 3: (Color online) Temperature dependent
M/H for rep-resentative members of the Sr − x Ca x Co P solid solution.(a): x = 0, 0 .
33 and 0 .
44; (b): x = 0 . .
49 (c): x = 0 . .
95 and 1 .
0. Inset: x = 0 .
85 (arrow shows the transitionpoint). no two-phase regions in the series; although the onsetof the a axis change is sudden, the changes are continu-ous. The anomalous lattice parameter variation clearlyreflects an unusual underlying change in electronic struc-ture that onsets suddenly when proceeding from the ucT(Sr) to the cT (Ca) phases. The lattice parameters ofthe Sr − x Ca x Fe P series (made by the same methodas the Co samples), by contrast, do not show similarlyanomalous variations. Figure 2 shows the detailed char-acterization of the crystal structures of Sr − x Ca x Co P series determined by SXRD. As x increases from 0 to 1,the P-P distance across the Sr − x Ca x intermediary layerdecreases substantially, from 3.3 ˚A to 2.4 ˚A. The P-Pseparation changes monotonically with composition. Instrong contrast both the Co-Co distance (equaling a/ √ ◦ to 121 ◦ ) vary in an un-expected fashion with composition. In addition to a dis-tinct onset of a dramatic change at x = 0.5, they displayanomalous maxima at x ∼ .
9. (Fig. 2)Figure 3 and 4 presents the magnetic properties forrepresentative members of the series. The whole seriesmanifests high-temperature Curie-Weiss (CW) behavior( χ ( T ) = C/ ( T − θ CW ) + χ ), with nearly parallel H/M curves (Fig. 4), indicating similar values of effective mo-ment ( µ eff ) per Co and differing values of Curie-Weiss Sr Ca x Co P x =0.3300.640.851.0H = 10 kOe H / M ( m o l C o / e m u ) T (K) M ( B / C o ) FIG. 4: (Color online) High-temperature Curie-Weiss behav-ior for representative members of the Sr − x Ca x Co P solidsolution. Inset: M ( H ) at 1.8 K. temperature ( θ CW ). At x = 0, SrCo P shows an en-hanced, temperature-dependent, paramagnetic suscepti-bility [ χ ( T )] with a broad maximum (Fig. 3 a) that istypical of nearly FM materials . As the Ca content x increases from 0 to 0 . χ ( T ) changes very little.For x > . χ ( T ) increases with increasing x , and thebroad maximum in χ ( T ) evolves to a more pronouncedfeature as x = 0 .
54 (Fig. 3 b). For x ≥ .
54, a sharpmaximum in χ ( T ) develops, indicating the appearance ofan AFM transition. The M ( H ) curves at 1.8 K in thiscomposition regime are consistent with an AFM groundstate (inset of Fig. 3 c). Both χ ( T ) and the AFM order-ing temperature ( T N ) increase as x increases, leading toatypical χ ( T ) behavior for 0 . ≤ x ≤ .
95 (Fig. 3 c andinset of b). The M ( H ) data in this composition regime at1.8 K (inset of Fig. 4) show small values of spontaneousmagnetization ( ∼ . µ B /Co for x = 0 . x > .
9, the M ( H ) data show no spontaneous magneti-zation, and χ ( T ) decreases dramatically with increasing x . The magnetic ordering temperature also drops withincreasing x , leading to T N = 87 ± x = 1. This T N for CaCo P is lower than previously reported (113 K) ,but is consistent with the rest of our series, possibly re-flecting a subtle difference of stoichiometry for samplesmade by different methods.Figure 5 (a) shows that the temperature-dependent re-sistivity data manifest a clear slope change due to mag-netic ordering for x ≥ .
64, but show no anomaly for x ≤ .
54. For x ≤ .
54, the low-temperature resistiv-ity data show FL behavior ( ρ ( T ) = ρ + AT ) belowa characteristic temperature (Fig. 5 b). The A valuesincrease and the characteristic temperatures decrease as x increases. The low-temperature specific heat data for x ≤ .
54 (inset of Fig. 5 b) show clear FL behavior x = 0.54 0.49 0.44 0 C P / T ( m J / m o l C o K ) T (K ) (a) Sr Ca x Co P - ( c m ) T (K) (b) - ( c m ) T (K ) FIG. 5: (Color online) (a) Temperature dependent resistivity,which has been normalized by the high temperature slope ofall resistivity data to that of SrCo P (the arrows show thetemperature where the slope changes); (b) low-temperatureresistivity versus T ; inset of a: low temperature specific heatfor x ≤ . ( C p = γ T + βT ), associated with very similar , inter-mediate magnitude γ values ( ∼ / molCoK ).The physical properties of Sr − x Ca x Co P are summa-rized in Fig. 6 and 7. The data show that the magneticproperties for the series are strongly correlated to thevariation of the structure (see Fig. 1 and 2). The valuesof µ eff vary in an S -shape manner, with minimum andmaximum values at x = 0 . .
85 respectively. θ CW varies from negative to positive on going from x = 0 . x = 1 .
0, showing a crossover from dominantly antiferro-magnetic to dominantly ferromagnetic interactions near x ∼ .
45, corresponding to the onset of the structural col-lapse, and showing a maximum value of approximately100 K at x ∼ . x increases, the values of the zero temperature suscep-tibility ( χ T =0 ) change relatively little for 0 ≥ x ≥ . x , becoming divergent for x > . χ T =0 then decreases again for x > .
9. The values of γ and A / , which are proportional to the effective mass ofthe quasi-particles in FL theory, change little for x < . χ T =0 .The electronic and structural phase diagram is sum-marized in Fig.7. The data show that the system evolves -100-50050100010203040 CWeff e ff ( B / C o ) x(b) T = ( e m u / m o l C o ) Sr Ca x Co P (a) C W ( K ) T=0 0 A ( m J / m o l C o K ) A / (( c m ) / T - ) FIG. 6: (Color online) Summary of physical properties forthe Sr − x Ca x Co P series (all lines are guides to the eye). (a)Effective moment ( µ eff )and Curie-Weiss temperature ( θ CW );(b) zero temperature susceptibility ( ∼ M/H at 1.8 K), γ and A / for the compositions showing FL behavior Sr Ca x Co P cTucT ? FM A F M AFMNFFL T ( K ) x FIG. 7: (Color online) the electronic and structural phase di-agram for Sr − x Ca x Co P . ucT and cT: uncollapsed tetrag-onal and collapsed tetragonal; NFFL: nearly ferromagneticFermi liquid; AFM and FM: antiferromagnetic and ferromag-netic order. from a NFFL ground state to an AFM ground statethrough a crossover composition regime near x = 0 . . < x ≤ .
9, the system manifests a FM-like ground state within which the the magnetic orderingtemperature is highest near x = 0 .
9. For x > .
9, anAFM ground state reappears.
IV. DISCUSSION AND CONCLUSION
The structure and physical property changes in theisoelectronic Sr − x Ca x Co P solid solution are driven bythe change in character of the P-P bond across the al-kaline earth layer. As previously described , the P-Pseparation in the CaCo P cT phase ( ∼ . − . In the ucT phaseSrCo P , the P-P distance (3.3 ˚A) is a non-bonding sep-aration, and thus each P can be considered formally asP − . A transition from a non-bonding to a bonding P-Psystem as x varies from 0 to 1 can therefore be antici-pated, but how that occurs for Sr − x Ca x Co P is sur-prising.As x first increases, from 0 to 0.4, both a and c de-crease slowly, indicating that the slowly increasing P-Phybridization has minimal impact on the electronic sys-tem. The replacement of Sr by smaller Ca in thiscomposition regime can therefore be considered as a sim-ple hydrostatic pressure effect. The P-P separation de-creases continuously on increasing x , and when it reachesa value shorter than 3.1 ˚A, near x = 0 .
4, the localizationof electrons in the P-P pairs begins to impact the distri-bution of electrons in the Co P layers, seen dramaticallyin the changes in the a axis. The unexpected behavioris that the slowly changing P-P bonding character, asseen in the continuously changing P-P bondlengths, in-duces a relatively sudden crossover of behavior of thein-plane Co-P electronic system. In this compositionregime, there appears to be a sudden onset to the redis-tribution of charge within those layers in response to thecontinuously increasing strength of the P-P bond. TheCo-P bondlength changes little across the series (2.23 ˚A-2.25 ˚A), indicating that the total charge in the Co P layers is constant. For x larger than the other criticalvalue, 0 .
9, with the P-P distance shorter than 2.5 ˚A, thesingle P-P bond appears to be fully formed, and both c and a decrease slowly with increasing x , again appear-ing to be a simple chemical pressure effect. The data inFigure 1 show that the Sr − x Ca x Fe P family does notshow a similarly dramatic structural variation. The ab-sence of this anomaly in the Fe case is interesting, sincethe P-P distances are similar to those in the Co system,varying from 3.4 ˚A to 2.6 ˚A, as one goes from Sr to Ca.The difference must therefore be due to the difference inelectron count in the T X layer.Although electronic structure calculations and furtherexperiments are needed to fully understand the phase di-agram, some conclusions can be drawn from our observa-tions. The two-dimensional, characteristic 122 structureof SrCo P indicates that its Stoner enhancement inter-action mainly occurs within the Co P layer. This is con-sistent with the fact that the Co moments in CaCo P ferromagnetically couple within the basal plane. For x < .
4, the nearly ferromagnetic FL ground state ofSr − x Ca x Co P is almost invariant (manifested in χ T =0 , γ and A / ), because the P-P distance across the Sr,Calayer has not reached a critical value at which the P-P bond becomes strong. For x > . µ eff decreases,reaches a minimum, and then increases again. If thehigh-temperature CW behavior in these nearly FM com-pounds is due to spin fluctuations associated with itin-erant electrons rather than local moments , then thechange of µ eff with x might indicate that itinerant elec-tron spin fluctuations are suppressed and local momentsstart to form in this composition regime. This process oflocal moment formation is correlated with the onset ofelectron localization in P-P bonds and the resulting re-distribution of charge within the CoP tetrahedra. Thisleads to an AFM ground state for 0 . < x < .
8. Giventhe positive values of θ CW , this AFM state is presumablyA-type. The correlation between the magnetic orderingtemperature and the Co-P-Co angle and Co-Co distanceindicates that both superexchange and direct exchangeare important; when the Co-P-Co angle and Co-Co sep-aration reach a maximum, a FM-like ground state ap-pears and the ordering temperature reaches its maximumvalue. Although details of the magnetic structure of thisFM-like ground state are unknown, AFM to FM transi-tions strongly correlated to the shape of CoX tetrahedrahave been seen in other Co compounds in this structuretype . For x > .
9, the P-P bond is fully formed and the Co-P-Co angle decreases with x , leading to an AFMground state with slightly lower T N .In conclusion, our experimental results reveal highlyanomalous changes in crystal structure within theSr − x Ca x Co P series, and correlated magnetic prop-erty changes due to the formation of P-P bonds acrossthe Sr,Ca layers that are induced by the substitution ofsmaller Ca for Sr . Due to the continuous nature ofthe structural changes, the Sr − x Ca x Co P system offersa unique avenue for exploring the evolution of pnictideelectronic structures from 2D-like to 3D-like as a conse-quence of lattice collapse in the 122 structure type. Fur-ther studies such as pressure-dependent magnetic prop-erties and neutron scattering on the compositions in thecritical regions, would be of interest. Acknowledgments
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