Protocol for the single-crystal growth of chromium-based RECrO₃ compounds

Summary

Chromium-based perovskites have gained significant interest for their magnetic and ferroelectric properties. Rare-earth orthochromates are versatile multiferroic materials in catalysts, thermistors, and non-volatile memory. We present a protocol for RECrO₃ crystal growth using a laser-diode floating-zone furnace. Steps include solid-phase calcination, sintering, and shaping. We provide detailed information on crystal growth parameters and Laue diffraction analysis. The successful growth of large orthochromate single crystals paves the way for exploring their intrinsic properties and potential applications.

For complete details on the use and execution of this protocol, please refer to Zhu et al.¹ and Zhu et al.²

Graphical abstract

Highlights

• •Grow orthochromate single crystals using a laser-diode floating-zone furnace
• •Laue diffraction analysis for large orthochromate single crystals
• •We provide a platform for the study of orthochromate single crystals

Publisher’s note: Undertaking any experimental protocol requires adherence to local institutional guidelines for laboratory safety and ethics.

Chromium-based perovskites have gained significant interest for their magnetic and ferroelectric properties. Rare-earth orthochromates are versatile multiferroic materials in catalysts, thermistors, and non-volatile memory. We present a protocol for RECrO₃ crystal growth using a laser-diode floating-zone furnace. Steps include solid-phase calcination, sintering, and shaping. We provide detailed information on crystal growth parameters and Laue diffraction analysis. The successful growth of large orthochromate single crystals paves the way for exploring their intrinsic properties and potential applications.

Before you begin

Zhu et al. have grown large single crystals of the family of RECrO₃ (RE = Y, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) compounds with a laser-diode floating-zone furnace.¹ Here, we present the single-crystal growth of YCrO₃ as a representative sample. This part tells the specific steps to prepare the raw materials and to use the equipment.

Preparation of raw materials

Timing: 30 min

Timing: 5 min (for step 1)

Timing: 20 min (for step 2)

• 1.For preparing 30 g YCrO₃, to calculate the required mass of Cr₂O₃ and Y₂O₃ according to the balanced chemical equation and formulae, i.e.,

$${\mathrm{Y}}_2{\mathrm{O}}_3+\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_3=2\mathrm{Y}\mathrm{C}\mathrm{r}{\mathrm{O}}_3$$

$$\mathrm{M}\mathrm{a}\mathrm{s}\mathrm{s}\mathrm{o}\mathrm{f}{\mathrm{Y}}_2{\mathrm{O}}_3=\frac{\mathrm{m}\mathrm{a}\mathrm{s}\mathrm{s}\mathrm{o}\mathrm{f}\mathrm{Y}\mathrm{C}\mathrm{r}{\mathrm{O}}_3}{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{a}\mathrm{r}\mathrm{m}\mathrm{a}\mathrm{s}\mathrm{s}\mathrm{o}\mathrm{f}\mathrm{Y}\mathrm{C}\mathrm{r}{\mathrm{O}}_3}\ast \frac{1}{2}\ast \mathrm{m}\mathrm{o}\mathrm{l}\mathrm{a}\mathrm{r}\mathrm{m}\mathrm{a}\mathrm{s}\mathrm{s}\mathrm{o}\mathrm{f}{\mathrm{Y}}_2{\mathrm{O}}_3$$

$$\mathrm{M}\mathrm{a}\mathrm{s}\mathrm{s}\mathrm{o}\mathrm{f}\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_3=\frac{\mathrm{m}\mathrm{a}\mathrm{s}\mathrm{s}\mathrm{o}\mathrm{f}\mathrm{Y}\mathrm{C}\mathrm{r}{\mathrm{O}}_3}{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{a}\mathrm{r}\mathrm{m}\mathrm{a}\mathrm{s}\mathrm{s}\mathrm{o}\mathrm{f}\mathrm{Y}\mathrm{C}\mathrm{r}{\mathrm{O}}_3}\ast \frac{1}{2}\ast \mathrm{m}\mathrm{o}\mathrm{l}\mathrm{a}\mathrm{r}\mathrm{m}\mathrm{a}\mathrm{s}\mathrm{s}\mathrm{o}\mathrm{f}\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_3$$

• 2.To weigh Cr₂O₃ and Y₂O₃ powder proportionally, then add 10% more Cr₂O₃ powder considering volatilization of Cr₂O₃ at 1100°C during solid phase calcination, mass as detailed in Table 1.

Table 1.

Summary of the required mass of Y₂O₃ and Cr₂O₃ compounds for the preparation of YCrO₃ crystals

Chemical	Molar mass (g/mol )	Mass (g)	Color
YCrO3	188.90	30.0000	Black
Y2O3	225.81	17.9307	White
Cr2O3	151.99	12.0690	Green
10% Cr2O3	151.99	1.2069	Green

Preparation of the equipment

Timing: 30 min

• 3.To confirm there is visually nothing wrong with the furnace and control box.
• 4.Supply cooling water before turning on the power to the furnace.
• 5.Turn on the power of the control box in order.
• 6.Open the front door to check if there are problems with laser parts, wiring, and cooling water tubes.

Key resources table

REAGENT or RESOURCE	SOURCE	IDENTIFIER
Chemicals, peptides, and recombinant proteins
Yttrium(III) oxide	Alfa Aesar	CAS: 1314-36-9
Gadolinium(III) oxide	Alfa Aesar	CAS: 12064-62-9
Terbium(III, IV) oxide	Alfa Aesar	CAS: 12037-01-3
Dysprosium(III) oxide	Alfa Aesar	CAS: 1308-87-8
Holmium(III) oxide	Alfa Aesar	CAS: 12055-62-8
Erbium(III) oxide	Alfa Aesar	CAS: 12061-16-4
Thulium(III) oxide	Alfa Aesar	CAS: 12036-44-1
Ytterbium(III) oxide	Alfa Aesar	CAS: 1314-37-0
Lutetium(III) oxide	Alfa Aesar	CAS: 12032-20-1
Chromium(III) oxide	Alfa Aesar	CAS: 1308-38-9
Software and algorithms
Origin2016	OriginLab Corporation	https://www.originlab.com/
OrientExpress	Ouladdiaf et al.3	https://neutronoptics.com/OrientExpress.html
Other
Laser-diode floating-zone furnace	Crystal Systems Corporation	LD-FZ-5-200W-VPO-PC-UM
X-ray diffractometer	Rigaku Corporation	Rigaku SmartLab 9 kW
Hydrostatic press system	Riken Seiki Co., Ltd.	HPTS-M-200
Laue crystal orientation system	Changzhou Yuhuan Optical Electronics Co., Ltd.	N/A
Vibratory micro mill PULVERISETTE 0	Fritsch GmbH	https://www.fritsch-international.com/sample-preparation/milling/ball-mills/details/product/pulverisette-0/
Chamber furnace	Nanjing Boyuntong Instrument Technology Co., Ltd.	KF1400
Oil-less pump/compressor	AutoBo Electronic Technology Limited	AP-1400C/V
Analytical balance	Precisa Gravimetrics AG	Series 321 LX 120 A SCS

Alternatives: Metal oxide powders with high purity are demanded to synthesize RECrO₃ compounds. High pure raw materials will effectively drive the quality of single crystals; brands of Alfa Aesar, Sigma and Acros all can work as backups. Also, alternative equipment from other brands can be used for measurements and characterizations. The software OrientExpress is for simulating the Laue patterns theoretically, which is irreplaceable.

Step-by-step method details

Solid state reaction

Timing: 4 days

Timing: 2 days (for step 1)

Timing: 2 days (for step 2)

The polycrystalline sample of YCrO₃ is synthesized with raw materials of Y₂O₃ and Cr₂O₃ by the solid state reaction (Figure 1). This section describes the details of solid phase calcination. Mill and mix the intermediate reaction product again to get homogeneous fine grains after each calcination.

• 1.
  First calcination.

  • a.
    Grind Cr₂O₃ and Y₂O₃ powders with a zircon ball mill for 1 h, then transfer the mixture into a crucible.
  • b.
    Put the crucible into a muffle furnace and heat it at 1100°C for 36 h with the same speed of temperature-up and temperature-down at 150°C/h in an air atmosphere. Naturally cool the furnace to 10°C–25°C when the temperature drops to 100°C.
  • c.
    Use X-ray powder diffraction to determine whether the resultant is polycrystalline with a single phase or not.
• 2.
  Second calcination.

  • a.
    Grind the intermediate reaction product after the first calcination with a zircon ball mill for 1 h, then transfer the product into the crucible used for the first calcination.
  • b.
    Put the crucible into a muffle furnace and heat it at 1200°C for 36 h with the same speed of temperature-up and temperature-down at 150°C/h in an air atmosphere. Naturally cool the furnace to 10°C–25°C when the temperature drops to 100°C.
  • c.
    Use X-ray powder diffraction to determine whether the resultant is polycrystalline with a single phase or not.

CRITICAL: A new crucible should be calcinated at above 1000°C to remove possible impurities and moisture before using.

CRITICAL: Use a dispensing spoon, brush and weighing paper to collect the powder to avoid potential loss of powder during the transfer between ball mill and crucible.

Figure 1.

Single-phase polycrystalline materials prepared by the solid state reaction

Shaping process with a hydrostatic press system

Timing: 2 days

This section describes how to prepare a feed rod and a seed rod.

• 3.
  Shaping process.

  • a.
    Compensate the evaporation of Cr.

    • i.
      Weigh the resultant product after the second calcination.
    • ii.
      Calculate the mole of the single-phase polycrystalline powder by the relative molecular mass of YCrO₃.
    • iii.
      Add 6% (0.72 g) Cr₂O₃ powder into the polycrystalline powder according to the corresponding stoichiometric ratios.
  • b.
    Mix and ball mill them for 1 h to micron-level polycrystalline particles.
  • c.
    Make uniform and straight rods.

    • i.
      Fill the mixture into a rubber tube and make it a cylindrical rod with aluminum cylindrical molds.
    • ii.
      Push and press the powder to make the raw material uniform and straight in the tube.
    • iii.
      Maintain a vacuum inside the rubber tube with a dry vacuum pump for 20 min during the shaping process.
  • d.
    Set the rubber tube vertically into the pressure vessel of a hydrostatic press system at 690 atm for 20 min.
  • e.
    Peel the rubber off the rod, then transfer it into a cylindrical crucible.
• 4.
  Sintering.

  • a.
    Put the crucible into a muffle furnace and heat it at 1300°C for 36 h with the same heating rate and cooling rate of 150°C/h in an air atmosphere. Naturally cool the furnace to 10°C–25°C when the temperature drops to 100°C.
  • b.
    Finally get the feed rod with a length of 6–15 cm and a diameter of 4–12 mm (Figure 2). Prepare the seed rod with a length of 3–5 cm and a diameter of 4–6 mm in the same way.

Note: Proper pressure is essential while squeezing the powder into the rubber tube. It is challenging to compact the powder and ensure a uniform diameter along the crystal growth direction.

Figure 2.

The feed rod shaped by a hydrostatic press machine with a length of ∼ 15 cm

Single-crystal growth with a laser-diode floating-zone furnace

Timing: 8−18 h

This section describes the optimized parameters of single-crystal growth with a laser-diode floating-zone furnace.

• 5.
  Set up the furnace.

  • a.
    Hang the feed rod and fix the seed rod in the convergence center of five uniform laser beams.
  • b.
    Then install a quartz tube that can resist high pressure.
  • c.
    Close the front door and do a final safety check.
• 6.
  Stable growth parameters.

  • a.
    Grow the single crystal in an argon atmosphere of 4–8 atm. Start 5 laser units, keep the feed rod and seed rod rotating in opposite directions at 20–32 rpm.
  • b.
    Adjust the growth rate and the laser power to attain a stable melting zone. The final growth condition is 72–82% of the laser unit’s maximum power.
  • c.
    The translational speed of the feed rod is 8–15 mm/h. The translational speed of the seed rod is 5–10 mm/h.
  • d.
    Finally obtain a large cylindrical single crystal that has a length of 5–10 cm and a diameter of 6–8 mm (Figure 3).

CRITICAL: Adjust growth parameters very carefully to make sure the melting zone will not collapse.

Figure 3.

Photographs of the grown single crystals^1,2 with a laser-diode floating-zone furnace

Laue diffraction study

Timing: 3 h

This section describes a way to confirm the quality of grown crystals.

• 7.Employ the Laue diffractometer to confirm the single-crystalline nature of the grown YCrO₃ crystals. The top panels of Figures 4A–4C show the Laue patterns of a YCrO₃ single crystal with the three axes perpendicular to the paper: a-axis (Figure 4A), b-axis (Figure 4B), and c-axis (Figure 4C). All patterns display symmetric and strong diffraction spots that demonstrate a high quality of the single crystal.
• 8.Theoretically simulate the Laue patterns with the software OrientExpress.³ The bottom panels of Figures 4A-4C further confirm that the single crystal is of good quality.

Figure 4.

Laue diffraction patterns of a YCrO₃ single crystal

(A–C) Laue patterns of a YCrO₃ single crystal (top panels) and the corresponding theoretical simulations (bottom panels).¹ The real space lattice vectors are marked in the bottom panels, and the crystallographic a axis (A), b axis (B), and c axis (C) are perpendicular to the paper.

Expected outcomes

This paper provides a specific method to grow single crystals of chromium-based RECrO₃ compounds. We can get large RECrO₃ single crystals with good quality, which will give fresh impetus to study orthochromates.

Limitations

The single-crystal growth of orthochromates is extremely difficult. First, the evaporation of Cr-based oxides is so intense like a thick haze. Second, single-crystal growth takes time and effort. The experimenter must carefully observe the growth state and optimize various parameters simultaneously.

Troubleshooting

Problem 1

Intense evaporation of chromium-based oxides (preparation of raw materials step 2).

Potential solution

Optimize the usage of Cr-based oxides. Add an extra 5–10% of Cr₂O₃ before the first calcination. Add 6–15% more to compensate the evaporation of Cr during the single-crystal growth.^4,5

Problem 2

Uniform straight rods with high density are crucial to the single-crystal growth. It is hard to make longer rods with high density (shaping process step 3).

Potential solution

To obtain straight and uniform rods with high density, the following steps are necessary.

• •Place two flat Al plates with a suitable diameter to make two ends of the rod flat. Then the loading force can transfer easily along the rod with a uniform diameter.⁶
• •Use an aluminum cylindrical mold to shape the rubber tube.
• •Put the rubber tube into a hydrostatic press system. Carry out the pressurization and the depressurization slowly.

Problem 3

Unstable molten zone (single-crystal growth step 6).

Potential solution

A prerequisite for smooth single-crystal growth is a stable melting zone. The grown crystals are affected by many growth parameters such as power, rotational speeds of feed and seed rods, as well as working atmosphere.

• •Rotate the upper shaft and lower shaft respectively. Confirm that both feed rod and seed rod are rotating around the center axis to avoid misalignment or oscillation of the feed rod when the two rods rotate in opposite directions. The oscillation of feed rod makes a molten zone unstable, which even may cause the collapse of the molten zone.
• •Temperature was estimated by predetermined relationship between applied electrical power and measured temperature.⁷ Excessive temperature can reduce the surface tension of the molten zone and lead to a loss of supercooling, affecting stability seriously. Tune the growth power when necessarily by observing the state of the melting zone during the crystal growth.
• •The relative rotation of the feed and seed rods is to stir the melts for the transport of energy and mass. When the rotational speed increases, the surface tension of the interface also increases, which is detrimental to the stability of the melting zone. Adjust the rotational speed according to the viscosity of the melts.
• •Adjusting the power and rotational speed plays a vital role in stabilizing the molten zone.

Problem 4

Lots of bubbles in the molten zone (single-crystal growth step 6).

Potential solution

The rods must be dense enough. Small amounts of bubbles can be eliminated by adjusting the relative speed of the upper and lower rods.

Problem 5

Lots of spontaneous nucleation sites (single-crystal growth step 6).

Potential solution

Employ a necking technique with a laser-diode floating-zone furnace to diminish the number of spontaneous nucleation sites. A steeper temperature gradient is more favorable for the nucleation and the formation of a stable growth state.^5,6 Initially, the seed rod lowers slightly faster than the feed rod, narrowing the melting zone and giving it an inverted trapezoid shape. Decreasing the translational speed of the feed rod makes the crystal radius small, which inhibits the formation of additional nuclei and speeds up the process from spontaneous nucleation to mononuclear growth.

Resource availability

Lead contact

Further information and requests for resources and reagents should be directed to and will be fulfilled by the lead contact, Prof. Dr. Hai-Feng Li (haifengli@um.edu.mo).

Materials availability

This long-term project produces a series of RECrO₃ (RE = Y, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) single crystals utilizing the innovative method described in the China Invention Patent (CN110904497B). We welcome potential collaborations.

Data and code availability

• •All data reported in this article will be shared by the lead contact upon request.
• •Code with instructions reported in this article will be shared by the lead contact upon request.
• •Any additional information required to re-analyze the data reported in this study is available from the lead contact upon request.