Stabilization of an ambient-pressure collapsed tetragonal phase in CaFe${}_2$As${}_2$ and tuning of the orthorhombic-antiferromagnetic transition temperature by over 70 K via control of nanoscale precipitates

We have found a remarkably large response of the transition temperature of CaFe${}_2$As${}_2$ single crystals grown from excess FeAs to annealing and quenching temperature. Whereas crystals that are annealed at 400${}^{ˆ}$C exhibit a first-order phase transition from a high-temperature tetragonal to a low-temperature orthorhombic and antiferromagnetic state near 170 K, crystals that have been quenched from 960${}^{ˆ}$C exhibit a transition from a high-temperature tetragonal phase to a low-temperature, nonmagnetic, collapsed tetragonal phase below 100 K. By use of temperature-dependent electrical resistivity, magnetic susceptibility, x-ray diffraction, Mössbauer spectroscopy, and nuclear magnetic resonance measurements we have been able to demonstrate that the transition temperature can be reduced in a monotonic fashion by varying the annealing or quenching temperature from 400${}^{ˆ}$ to 850${}^{ˆ}$C with the low-temperature state remaining antiferromagnetic for transition temperatures larger than 100 K and becoming collapsed tetragonal, nonmagnetic for transition temperatures below 90 K. This suppression of the orthorhombic-antiferromagnetic phase transition and its ultimate replacement with the collapsed tetragonal, nonmagnetic phase is similar to what has been observed for CaFe${}_2$As${}_2$ under hydrostatic pressure. Transmission electron microscopy studies indicate that there is a temperature-dependent width of formation of CaFe${}_2$As${}_2$ with a decreasing amount of excess Fe and As being soluble in the single crystal at lower annealing temperatures. For samples quenched from 960${}^{ˆ}$C there is a fine (of order 10 nm) semiuniform distribution of precipitate that can be associated with an average strain field, whereas for samples annealed at 400${}^{ˆ}$C the excess Fe and As form mesoscopic grains that induce little strain throughout the CaFe${}_2$As${}_2$ lattice.

Figure 1

Temperature-dependent (a) magnetic susceptibility and (b) normalized electrical resistivity of CaFe${}_2$As${}_2$ for three differently prepared single crystals: (squares) Sn grown, (circles) as grown (quenched from 960${}^{ˆ}$C) from FeAs, and (triangles) FeAs grown, annealed for 1 week at 400${}^{ˆ}$C. Note: when the as-grown sample from FeAs melt was cooled below the transition temperature near 90 K it shattered, making further lower-temperature resistivity measurements impossible.

Figure 2

Temperature-dependent magnetic susceptibility and normalized electrical resistivity of as-grown CaFe${}_2$As${}_2$ single crystals annealed for 24 h at temperature, $T_a$. Susceptibility data in (a) have been offset from each other by an integer multiple of $1.5\times {10}^{-4}$ emu/mole for clarity. Data for a 1-week anneal of a whole batch at 400${}^{ˆ}$C is shown for comparison. The resistivity data (b) for the as-grown sample could not be measured below the transition temperature due to sample breakage, but for the sample annealed at 850${}^{ˆ}$C resistivity could be measured through the transition.

Figure 3

Temperature-dependent magnetic susceptibility of (a) as-grown CaFe${}_2$As${}_2$ single crystals annealed at 450${}^{ˆ}$C for representative times and (b) as-grown CaAs${}_2$Fe${}_2$ single crystals that have been annealed for a week at 500${}^{ˆ}$C and then annealed at 800${}^{ˆ}$C for representative times. Data in both panels have been offset from each other by an integer multiple of $1\times {10}^{-4}$ emu/mole for clarity.

Figure 4

Temperature-dependent (a) magnetic susceptibility and (b) normalized electrical resistivity of as-grown CaFe${}_2$As${}_2$ single crystals that were first annealed for a week at 400${}^{ˆ}$C and then annealed for 24 h at temperature, $T_a$. Susceptibility data in (a) have been offset by an integer multiple of $1\times {10}^{-4}$ emu/mole from each other for clarity.

Figure 5

(a) Transition temperature-annealing temperature ($T^{*}$-$T_a$) phase diagram. Open symbols are inferred from resistivity data and filled symbols are inferred from susceptibility data. (Star) The as-grown samples (quenched from 960${}^{ˆ}$C data and are also shown as 20${}^{ˆ}$C anneals); (squares) as-grown samples that have been annealed for 24 h at $T_a$ and quenched to room temperature; (circles) as-grown samples that were first annealed for a week at 400${}^{ˆ}$C and then annealed for 24 h at $T_a$ and quenched to room temperature; and (diamonds) as-grown samples that have been annealed for a week as whole, unopened batches at $T_a$. (b) $T^{*}$ as a function of pressure from Ref. [9] and $T^{*}$ as a function of $T_a$ for 400${}^{ˆ}$C $⩽T_a⩽{960}^{°}$C.

Figure 6

Temperature-dependent (a) magnetic susceptibility and (b) normalized electrical resistivity and their respective temperature derivatives for an as grown crystal annealed at 500${}^{ˆ}$C for 24 h. $T^{*}$ is inferred from the temperature of the extremum in the derivative. Full width at half maximum of derivative is used to define error bars shown in Fig. 5. Values of the jump in magnetic susceptibility (maximum and minimum values) and resistivity are inferred as shown.

Figure 7

Size of jump in susceptibility and normalized resistivity as a function of transition temperature for FeAs-grown CaFe${}_2$As${}_2$ crystals annealed at temperatures shown in Figures 1, 2, 3, 4. Room-temperature resistivity of samples with $T^{*}$ values of $~$ 90, 130, and 170 K all fall within the 3.75$\pm$0.75 $\mathrm{m}\Omega$ cm range.

Figure 8

Values for (a) the $c$-lattice paramter, (b) $a$-lattice parameter, and (c) unit cell volume as a function of temperature for an unannealed FeAs flux-grown CaFe${}_2$As${}_2$ sample determined from high-energy x-ray diffraction measurements. The open squares denote the results of measurements performed on a polycrystalline sample under applied hydrostatic pressure of 0.63 GPa from Ref. 7. The inset to the middle panel is the image of the $(220)$ diffraction peak taken from the two-dimensional x-ray detector as described in the text. Note the absence of any splitting that would signal a transition to an orthorhombic phase [the two open circles illustrate the expected distance between split Bragg peaks due to the “usual” orthorhombic distortion].

Figure 9

${}^{57}\mathrm{Fe}$ Mössbauer spectra of an $\mathit{ab}$-plane single-crystal mosaic of samples (from left) annealed at 500${}^{ˆ}$C, annealed at 700${}^{ˆ}$C, and as-grown (quenched from 960${}^{ˆ}$C). In each case the upper spectrum was taken at 295 K, whereas the lower spectrum was taken at 10 K or 5 K. Only the as-grown sample shows no magnetic ordering at base temperature. The solid lines are fits as described in the text.

Figure 10

Temperature dependence of the magnetic hyperfine fields ($B_{\mathrm{hf}}$) for the two annealed samples shown in Fig. 9. The solid lines are fits to Brillouin functions (as described in the text) used to estimate the Néel temperatures of the low-temperature, antiferromagnetic, orthorhombic form of each sample. In both cases the samples undergo a first-order structural transition on warming effectively truncating the $B_{\mathrm{hf}}(T)$ data.

Figure 11

${}^{75}\mathrm{As}$ NMR spectra measured at $f=51$ MHz for (a) ${400}^{°}$C annealed CaFe${}_2$As${}_2$ crystal and (b) for the as-grown CaFe${}_2$As${}_2$ crystal. Black and blue lines are observed and simulated spectra, respectively. Expected lines above 9 T are not measured due to the limited maximum magnetic field for our SC magnet. (c) ${}^{75}\mathrm{As}$ NQR spectrum at $T=4.2$ K and $H=0$ T.

Figure 12

(a) TEM micrograph of the as-grown and quenched sample. The fine tweedlike pattern is due to thin platelets of a second phase which are coherent with the matrix and parallel the $\{h,0,0\}$ planes. The inset on the right is the SADP from this image area showing expected lattice reflections for a $[0,0,1]$ zone axis for the 122 compound. Note the absence of streaking or extra reflections as discussed in the text. (b) TEM micrograph of the sample annealed at 500${}^{ˆ}$C for 1 week at the same magnification and zone axis orientation as the as-grown sample above. Note that the rectangular precipitates are oriented along the same $\{h,0,0\}$ planes as the finer features above. The inset on the left shows the SADP of the matrix and precipitates while the right inset is a CBED of only the precipitate phase showing nearly the same orientation and $d$ spacing as the matrix 122 phase.