Introduction
Ice isn't just the solid water you find in your freezer or a glacier. Since 1900, scientists have identified more than 20 distinct phases of ice, many formed only under extreme conditions. The list includes surprising varieties like hot ice and even ice that conducts electricity. At its core, ice is any solid, crystalline phase of water—meaning its molecules are arranged in a repeating, organized pattern. This guide takes you through the process physicists use to discover and characterize these complex ice forms.
What You Need
- High-pressure apparatus (e.g., diamond anvil cell or multi-anvil press) capable of exceeding 10 GPa
- Cryogenic cooling system to achieve temperatures as low as 80 K (−193°C)
- Water sample (ultrapure, deionized) in a sealed container
- X-ray diffraction or neutron scattering equipment for crystal structure analysis
- Spectroscopy tools (Raman or infrared) to monitor molecular vibrations
- Pressure and temperature sensors with real-time data logging
- Safety gear: blast shields, cryogen gloves, and proper ventilation
Step-by-Step Guide
Step 1: Understand the Basics of Ice Phases
Before diving into discovery, familiarize yourself with the known ice phases. All solid water phases are crystalline, but their molecular arrangements differ based on temperature and pressure. For example, ordinary ice (Ih) has a hexagonal structure, while high-pressure phases like Ice VII are cubic and dense. Knowing the existing 20+ phases—including exotic ones like hot ice (Ice VII at high temperature) and conducting ice (where protons move freely)—is essential to recognize something truly new.
Step 2: Set Up the High-Pressure Environment
To push water into exotic phases, you need extreme pressure. Use a diamond anvil cell: two diamonds squeeze a tiny sample between them. Calibrate the pressure by measuring the fluorescence of a ruby chip placed inside the cell. Ensure the diamonds are perfectly aligned to avoid uneven stress. For multi-anvil presses, stack tungsten carbide anvils in a hydraulic frame. Pre-compress the gasket to create a stable sample chamber.
Step 3: Introduce the Water Sample
Place a small drop of ultrapure water (avoiding contaminants that could seed unwanted crystal growth) inside the gasket hole. Seal the cell carefully to prevent leaks. For cryogenic runs, pre-cool the entire assembly to avoid vaporization. The sample volume should be as small as practical to ensure uniform pressure and temperature distribution—typically tens to hundreds of micrometers in diameter.
Step 4: Apply Pressure and Temperature
Gradually increase the pressure using the manual or automated screw mechanism. Monitor the pressure reading from the ruby fluorescence. Simultaneously, adjust the temperature using a resistive heater (for high T) or a cryostat (for low T). Many new phases appear at combinations beyond 1 GPa and 200 K. Ramp conditions slowly (e.g., 0.01 GPa/min) to allow phase transitions to complete without trapping metastable states.
Step 5: Observe Phase Changes with Spectroscopy
While changing conditions, use Raman spectroscopy or infrared microscopy to detect changes in molecular bonding. In a phase transition, the O–H stretching frequencies shift, and new lattice modes appear. For example, when Ice Ih transforms to Ice II, the Raman spectrum shows distinct peak splitting. Record spectra at regular intervals. Look for abrupt changes that suggest a new crystalline form—especially if the spectrum doesn't match any known phase.
Step 6: Determine the Crystal Structure
Once a potential new phase is detected, perform X-ray diffraction (using synchrotron radiation for high resolution) or neutron diffraction (to locate hydrogen atoms). Collect diffraction patterns at multiple angles and compare them with computational predictions. Index the peaks to determine lattice symmetry and unit cell dimensions. For very complex phases, single-crystal diffraction may be needed—grow a single crystallite by slow compression near the phase boundary.
Step 7: Identify and Characterize the New Phase
Analyze the diffraction data to solve the structure. If the atomic arrangement is unique and not previously reported, you've discovered a new ice phase. Characterize its properties: density, thermal stability, electrical conductivity (if applicable), and proton ordering. Run simulations to confirm the structure's mechanical stability. Publish the results, giving the phase a Roman numeral designation (e.g., Ice XXI).
Tips for Success
- Start with known phases to calibrate your setup. Reproduce the formation of Ice VI or Ice VII before hunting for new ones.
- Collaborate with theorists who can predict likely phase boundaries using computational chemistry (e.g., density functional theory). This saves experimental time.
- Control for dynamic effects—some phases only exist as transient states. Fast measurements (millisecond X-ray pulses) can capture them.
- Document everything; tiny variations in sample purity or pressure ramp rate can lead to different phases.
- Safety first: High-pressure cells can explode. Always use blast shields and remote operation when possible.