Question: what is the purest form of iron called
[{'answer': 'pure , single crystals', 'provenance': [{'wikipedia_id': '14734', 'title': 'Iron', 'start_paragraph_id': 132, 'start_character': 184, 'end_paragraph_id': 132, 'end_character': 208, 'bleu_score': 1.0, 'section': 'Section::::Applications.:As structural material.:Mechanical properties.\n'}]}]